THE  LIBRARY 

OF 

THE  UNIVERSITY 

OF  CALIFORNIA 

RIVERSIDE 

GIFT  OF 


An  Anonymous  Donor 


MliNERALS  IN  ROCK  SECTIONS 


THE    PRACTICAL    METHODS   OF 

IDENTIFYING   MINERALS  IN    ROCK    SECTIONS. 

WITH    THE    MICROSCOPE 


ESPECIALLY    ARRANGED    FOR    STUDENTS    IN    TECHNICAL 
AND    SCIENTIFIC    SCHOOLS 


LEA    McILVAINE   LUQUER,   C.E.,  Ph.D. 

I  ■  ^  ■ 

Adjunct  Professor  of  Mineralogy, 
Columbia    University,   N^eiv    York    City 


REVISED   EDITION 


NEW  YORK 

D.  VAN    NOSTRAND    COMPANY 

1905 


Copyright,   1905 
By   lea   MclLVAINE   LUQUER 


Prfss  of 

The  New  Eba  Printing  Company 

LA^CASTEB,  Pa. 


PREFACE  TO  REVISED  EDITION. 


In  preparing  the  revised  edition,  Chapters  I.  and  IV.  have  been 
rewritten  and  enlarged  and  the  part  relating  to  the  determination  of 
the  plagioclases  has  been  greatly  amplified.  Many  additions  have 
also  been  made  to  Chapter  III  and  the  Becke  method,  for  the 
determination  of  the  relative  indices  of  refraction  of  minerals,  has 
been  given  in  detail. 

Some  new  and  useful  tables  have  been  introduced  ;  as  tables  of 
refractive  indices  (mean)  and  double  refraction  (maximum).  A 
diagram  has  also  been  added,  showing  the  relation  existing  be- 
tween strength  of  double  refraction,  interference  colors  and  thick- 
ness of  section. 

Professor  E.  Weinschenk's  admirable  text-book,  "  Die  Gesteins- 
bildenden  Mineralien,  Freiburg,  1901,  has  been  specially  referred 
to,  and  the  tables  of  refractive  indices  and  double  refraction  have 
been  compiled  from  Weinschenk's  new  Tables. 

Many  new  cuts  have  been  added,  among  them  being  semi-ideal 
drawings,  showing  typical  outlines  of  crystal  sections,  cleavage, 
optical  orientation,  etc.  In  describing  the  "  Usual  Appearance  in 
Sections  "  of  a  mineral,  it  is  of  course  only  possible  to  mention  the 
usual  crystal  form  in  which  the  mineral  occurs  in  a  rock.  The 
crystal  may  be  cut  in  any  way  by  the  plane  of  the  section  ;  but  a 
general  knowledge  of  the  crystal  forms  will  furnish  an  idea  as  to 
'the  outline,  etc.,  that  the  mineral  may  show  in  the  section. 

Lea  McI.   Luouer. 

Department  of  Mineralogy, 

Columbia  University,   New  York,  July,  1905. 


PREFACE  TO  FIRST  EDITION,  1898. 


The  identification  of  minerals  in  rock  sections  with  the  micro- 
scope, including  as  it  does  a  knowledge  of  optical  mineralogy,  is 
often  difficult  for  beginners.  This  may  be  due  to  the  fact  that 
most  of  the  publications  on  this  subject  are  quite  elaborate  in  their 
nature  and  in  either  French  or  German.  While  detailed  descrip- 
tions are  very  necessary,  and,  in  fact,  indispensable  for  advanced 
inv^estigation,  they  are  apt  to  prove  cumbersome  and  confusing  at 
first.  For  these  reasons  this  text-book  has  been  prepared  by  the 
writer,  with  a  view  of  putting  before  the  student  only  those  facts 
which  are  absolutely  necessary  for  the  proper  recognition  and 
identification  of  the  common  minerals  in  rock  sections.  The  foot- 
notes refer  the  student  to  standard  publications,  in  which  are  given 
details  of  the  methods  and  investigations  outlined  in  the  text.  An 
elementar}^  knowledge  of  crystallography  and  mineralogy  is  almost 
indispensable  and  is  here  assumed. 

The  microscopic  and  optical  characters  of  the  minerals  are  re- 
corded in  the  usual  order  in  which  they  would  be  observed  with  a 
petrographical  microscope.  Nearly  all  the  rock-forming  minerals 
become  transparent  in  thin  sections  ;  but  when  opaque,  attention 
is  called  to  the  fact  and  the  characters  are  recorded  as  seen  with 
incident  light.  White  light  is  assumed  to  be  used,  unless  other- 
wise stated.  The  interference  colors  recorded  in  all  cases  are 
those  given  by  very  thin  sections  of  0.03  mm.  in  thickness. 

The  order  followed  for  the  minerals  is  essentially  that  of  Rosen- 
busch  (based  on  the  symmetry  of  the  crystalline  form),  with  a  few 
exceptions  made  for  convenience,  such  as  placing  pyrrhotite  after 
pyrite  and  zoisite  after  epidote.  The  statements  regarding  the 
occurrences  of  minerals  in  the  common  rock-types  have  been  taken 
mainly  from  Les  Mineraux  des  Roches,  by  Levy  and  Lacroix. 

The  terms  axes  and  directions  of  elasticity,  used  throughout 
this  book,  are  very  commonly  employed  in  petrographical  litera- 
ture of  the  present  time.  These  axes  and  directions  should  prob- 
ably more  correctly  be  called  axes  and  directions  of  vibration  or 

V 


VI 


PREFACE  TO  FIRST  EDITION,   iSgS. 


extinction.  The  reasons  for  or  against  the  elastic  condition  of  the 
"  ether  "  are  of  more  interest,  however,  to  the  physicist  than  to  the 
petrographer. 

An  optical  scheme  is  appended,  with  the  minerals  grouped  ac- 
cording to  their  common  optical  characters. 

The  writer's  thanks  are  due  to  Dr.  A.  J.  Moses,  Professor  of 
Mineralogy,  and  to  Mr.  J.  F.  Kemp,  Professor  of  Geology,  for 
kind  suggestions  offered  during  the  preparation  of  this  book. 

L.   McI.   L. 

Department  of  Mineralogy, 

Columbia  University,  N.  Y.  City, 
October,   i8q8. 


TABLE  OF  CONTENTS. 


PAGE. 

Conventions  and  Abbreviations ix 

CHAPTER  I. 
Introductory  Optics  for  Optical  Mineralogy. 
Ordinary  Light.  —  Plane  Polarized  Light.  —  Effects  Produced  by 
Crystal  Sections  on  Transmitted  Light.  —  Amorphous 
Bodies.  —  Isotropic  Crystals.  —  Anisotropic  Crystals.  — 
Double  Refraction. — Uniaxial  Crystals. — Biaxial  Crys- 
tals. —  Principal  Vibration  Directions.  —  Axial  Angle.  — 
Dispersion i 

CHAPTER  II. 
Petrographical   Microscope. 
Reflector.  —  Polarizer.  —  Nicol  Prism.  — Condensing  Lens.  — Ro- 
tating Stage. —  Objectives. — Analyzer. — Eye-pieces.     .      .      7 

CHAPTER  III. 

Investigation  of  Microscopic  and  Optical  Characters  of 

Minerals. 
Opaque  Minerals. 
Transparent  ISIinerals : 

With  transmitted  light :  Form,  Color,  Index  of  Refrac- 
tion and  Relief,  Becke  Alethod,  Cleavage,  Fracture,  In- 
clusions. 

With  polarized  transmitted  light :  PleocJiroisni. 
With  crossed  nicols :  Isotropic  Character,  Anisotropic 
Character,  Interference  Colors,  Extifiction  and  Extinction 
Angles,  Vibration  Directions  of  Faster  and  Slower  Rays, 
Order  of  Interference  Color,  Strength  of  Double  Refrac- 
tion,  Determination  of  Minerals  and  Thickness  of  Section, 
Structure. 

With  convergent  light :  Uniaxial  Interference  Figures 
and  Optical  Character,  Biaxial  Interference  Figures  and 
Optical  Character,  Determination  of  Axial  Angle,  Distinc- 
tions between  Ortho7-hombic,  Monoclinic  and  Triclinic  Sec- 
tions. 
Resume  of  Uses  of  Parallel  and  Convergent  Light 13 


vm  TABLE  OF   COXTENTS. 

CHAPTF:R    IV. 

Microscopic  and  Optical  Characters  of  Minerals. 
Amorphous    Minerals. — Isometric    Minerals. — Tetragonal    Min- 
erals. —  Hexagonal    Minerals.  —  Orthorhombic    Minerals. 

—  Monoclinic  Minerals. — Triclinic  Minerals. — Mineral 
Aggregates 49 

CHAPTER    V. 
Methods  of  Preparing  Sections. 
Cutting  and  Grinding  Machines. — Saws.  —  Cutting. — Grinding 
Plates  or  Laps.  —  Cementing.  —  Grinding.  —  Mounting.  — 
Cleaning  and  Finishing.  —  Convenient  Apparatus  for  work.    11 1 

CHAPTER    VI. 

Chemical  and  Mechanical  Tests. 
Chemical  Tests  on  Crystal   in  Section  ;     Carbonates,   Gelatinizing 
Silica.  —  Etched  Figures.  —  Heating  Section  to  Redness. 

—  Methods  of  Isolating  Crystals  or  Fragments  for  Testing  ; 
Specific  Gravity  Separation,  Electro-magnetic  Separation, 
Chemical  Separation.  —  Micro-Chemical  Reactions  ;  Bor- 
ic hy^s  Method,  Behren' s  Method,  Special  Tests     .      .      .      .123 

APPENDIX. 

Brief  Scheme  of  Classification  into  Systems  by  Optical  Determina- 
tions.—  Tables  of  Double  Refraction  (maximum)  and 
Indices  of  Refraction  (mean).  — Diagram,  showing  relation 
between  strength  of  double  refraction,  interference  colors 
and  thickness  of  section.  — Order  of  Consolidation  of  the 
Constituent  Minerals  in  Plutonic  Rocks.  —  Optical  Scheme 
with  Special  Introduction 135 

Index 143 


CONVENTIONS    AND    ABBREVIATIONS. 

Elongation  relates  to  the  appreciable  extension  often  shown  by  the 
crystal  section.  A  crystal  of  long  prismatic  habit,  cut  about  parallel  to 
the  c  axis  would  show  marked  elongation  ;  while  a  tabular  crystal  (like 
mica)  would  show  elongation  if  cut  at  right  angles  to  the  tabular  faces. 
At  times,  of  course,  no  elongation  is  appreciable,  as  in  the  case  of  gran- 
ular or  broken  crystals  or  where  the  cross-section  is  essentially  square  or 
octagonal.  Very  often  the  relation  of  the  cleavage  is  given  to  the 
elongation  and  also  to  the  directions  a'  and  c',  which  makes  it  possible 
to  test  for  a'  and  c'  even  when  no  marked  elongation  can  be  observed. 

a'  =  The  assumed  direction  of  the  ether  vibrations  of  the  faster  ray  in 
the  given  section.* 

c'  =  The  assumed  direction  of  the  ether  vibrations  of  the  slower  ray 
in  the  given  section. 

(-[-)  =  Optical  character  positive. 

(  —  )  =  Optical  character  negative. 

II  =  Parallel  to. 

(j'  —  a)  =  The  difference  between  the  indices  of  refraction  of  the 
slowest  and  fastest  rays,  respectively,  transmitted  by  the  crystal,  and 
indicates  in  decimals  the  relative  strength  of  the  double  refraction. 

n'  =  The  mean  index  of  refraction  ;  hence 

a  -\-  i3  -j-  Y        e  -{-  2io 
= or . 


a,  h  and  c  relate  to  the  crystallographic  axes  commonly  represented 
by  these  letters. 

2^  =  the  apparent  axial  angle  measured  in  air,  2Fbeing  the  true 
angle. 

Bx^.  =  The  acute  bisectrix. 

Ax.  pi.  =  The  axial  plane,  /.  e.,  the  plane  containing  the  two  "  optic 
axes. ' ' 

Iddings'  Rosenbusch  =  Iddings'  translation  of  Mikroskopische  Phys- 
iographic der  petrographisch  wichtigen  Mineralien,  von  Rosenbusch, 
1900  Edition. 


*  Often   called  a  direction  of  maximum   elasticity,  t'  being  a  direction  of  minimum 
elasticity. 

ix 


Minerals  \n  Rock  Sections. 

CHAPTER  I. 

Introductory  Optics  for  Optical  Mineralogy. 

The  object  of  this  introduction  is  merely  to  give  a  practical  dis- 
cussion of  elementary  optics,  as  applied  to  optical  mineralogy,* 
and  no  elaborate  discussion  of  this  important  subject  will  be  at- 
tempted. The  explanations  will  be  made  as  simple  as  possible, 
and,  in  most  cases,  only  the  optical  phenomena  will  be  described 
without  entering  into  a  theoretical  discussion  as  to  the  cause  of 
these  phenomena. 

Li£-/i/  is  transmitted  by  vibrations  of  the  "ether,"  taking  place 
at  right  angles  to  the  direction  of  transmission. 

Ordijiary  light  is  light  with  the  ether  vibrations  in  all  possible 
directions,  the  path  described  by  any  particle  of  ether  constantly 
changing. 

Plane  polarized  light  is  simply  light  with  the  ether  vibrations  all 
parallel  to  one  plane  passing  through  the  direction  of  transmission. 

By  experiment  it  has  been  proved  that  there  exists  a  very  close 
relation  between  the  optical  properties  of  crystals  and  their  other 
physical  properties,  such  as  form,  color,  transmission  of  heat,  etc. 
Therefore  it  is  often  possible,  by  a  careful  optical  investigation  of 
a  crystal  section,  to  determine  important  crystallographic  facts^ 
even  in  the  absence  of  any  distinct  outline. 

The  Effects  Produced  by  Crystals  on  Transmitted  Light. 

Consider  that  a  series  of  optical  tests  are  made  on  all  possible 

*  For  a  more  complete  discussion  of  optics,  in  connection  with  Optical  Mineralogy, 
the  student  is  referred  to  A.  J.  Moses'  Characters  of  Crystals,  p.  85,  et  seq. ;  Moses'  & 
Parsons'  Mineralogy,  Crystallography  and  Blowpipe  Analysis,  Chap.  XVI.,  3d  Ed., 
1904;  Miers'  Mineralogy,  1902  ;  L.  Fletcher's  Optical  Indicatrix,  etc.,  1892  ;  Groth's 
Physikalische  Krystallographie,  3d  Ed. ;  'Ko'i,&\^\xsc!a'?,  Mikroskopische  Physiographic, 
3d  Ed.  (new  Ed.  in  preparation)  and  Iddings'   Translation  of  Rosenbiisch,  4th  Ed. 

I  I 


2  INTRODUCTORY   OPTICS. 

sections  *  of  crystals  in  the  six  systems,  and  the  manner  in  which 
these  crystals  affect  transmitted  Hght  ascertained. 

Isotropic  Crystals  :  It  will  be  found  that  all  sections  of  Iso- 
metric crystals  transmit  light  with  equal  velocity  in  all  directions ; 
that  is,  the  crystals  are  optically  equivalent  in  all  directions  and, 
hence,  can  produce  no  double  refraction.!"  In  these  crystals  any 
section,  however  cut,  will  transmit  all  the  rays  of  light,  incident  to 
the  surface  at  right  angles,  with  no  change.  J  The  same  is  true 
of  Amorplious  bodies,  glass,  etc.,  unless  they  have  been  subjected 
to  strains  or  peculiar  conditions  during  cooling.  A  single  image 
is  seen  through  these  isotropic  sections. 

Anisotropic  Crystals :  It  will  also  be  found  that  nearly  all  sec- 
tions (the  exceptions  being  given  later)  of  cr)'stals  in  the  remaining 
five  systems,  produce  quite  a  different  effect  on  transmitted  light. 
In  these  crystals  the  velocity  of  transmission  of  light  varies  with 
the  vibration  direetion  of  the  light  rays.  This  property,  called 
double  refraction, %  seems  to  result  from  the  power  of  resolving  a 

ray  of  ordinary  light,  with  ether  vibra- 
tions in  all  directions,  into  two  rays 
with  ether  vibrations  in  planes  at  right 
angles  to  each  other  ;  the  two  resulting 
rays  tra\'ersing,  usually,  divergent  paths 
in  passing  through  the  section. 
P  ■  The    mineral    calcite   (Iceland    spar) 

exhibits  this  property  to  a  marked  de- 
gree, and  in  certain  sections  will  show  a  double  image.  Fig.  i. 
That  the  vibration  directions  of  the  two  doubly  refracted  rays  are 
in  planes  at  right  angles  to  each  other,  can  be  easily  proved 
by   using   a    nicol    prism  ||.       In    most    cases    the    separation    of 

*  These   sections  are  supposed  to  have   plane  parallel  faces,  such    being  the  case  in 
ordinary  practice,  and  to  be  examined  with  parallel  perpendicularly  incident  light. 

fit  is  interesting  to  remember  in  this  connection  that  in  the  isometric  system  there  is 
also  the  greatest  possible  symmetry  of  "form." 

J  A.  J.  Moses,  Characters  of  Crystals,  pp.  85-97. 

\  For  this  branch  of  optical  physics  see  A.    J.    Moses,  Characters  of  Crystals,   pp. 
97-100. 

II  This  can  be  demonstrated  by  using  a  nicol  and  a  plate  of  calcite  which  shows  a 
double  image.  If  the  nicol  is  held  between  the  calcite  plate  and  the  observer's  eye  it 
can  be  so  adjusted  that  only  one  image  is  seen.  If  now  the  nicol  is  revolved  90°  the 
first  image  will  disappear  and  the  other  image  alone  will  be  seen. 


EFFECTS  OF  CR  YSTALS  ON  TRANSMITTED  LIGHT.         3 

the  two  images  is  so  slight  as  not  to  be  perceived  by  the  eye,  and 
the  practical  method  of  testing  a  crystal  section  for  double  refrac- 
tion will  be  given  later,  p.  26. 

The  crystals  that  show  double  refraction  are  further  divided 
into  two  groups,  uniaxial  and  biaxial  : 

(i)  Uniaxial,  or  those  in  which  the  optical  characters  are  sym- 
metrical to  o)ic  direction,  called  an  optic  axis.  This  optic  axis  is 
the  crystallographic  vertical  axis,  c  ;  and  parallel  to  this  direction 
there  is  a  single  value  only  for  the  light  velocity  and  no  double 
refraction  takes  place  *.  Hence  any  section  parallel  to  the  base 
(OP,  001),  being  at  right  angles  to  the  optic  axis,  acts  like  a  sec- 
tion of  an  isotropic  crystal  and  transmits  all  the  perpendicularly 
incident  rays  of  light  with  no  change.  In  any  other  section  double 
refraction  takes  place  and  it  can  be  proved  by  using  a  nicol  prism 
that  the  two  rays  have  ether  vibrations,  one  in  the  plane  passing 
through  the  incident  ray  and  the  c  axis  of  the  crystal,  and  the 
other  in  a  plane  at  right  angles  thereto,  hence  in  the  basal  plane. 
This  latter  ray,  which  has  a  constant  velocity,  is  called  the  ordi- 
nai'v  ray  0 ;  and  the  other  ray,  with  velocity  varying  with  the 
inclination  of  the  section  to  r  is  called  the  extraordinary  ray  E.  f 

The  vibration  directions  are  either  parallel  or  symmetrical  to 
cleavage  cracks  and  crystal  outlines.  In  sections  parallel  to  the 
optic  axis,  the  two  doubly  refracted  rays  have  the  maximum  dif- 
ference in  velocity  of  transmission,  and  hence  their  vibration  direc- 
tions are  called  principal  vibration  directions  %  and  the  plane  con- 
taining them  an  optical  principal  section.  In  these  sections  the 
refractive  index  of  the  ray  vibrating  parallel  to  c  (extraordinary 
ray)  is  denoted  by  e,  and  that  of  the  ray  vibrating  parallel  to  the 
basal  plane  (ordinary  ray)  by  &).§ 

*  In  some  cases  a  peculiar  form  of  double  refraction  does  take  place  parallel  to  this 
direction,  as  in  the  circular  polarization  of  quartz  and  cinnabar  ;  but  in  very  thin  sec- 
tions these  results  are  not  noticed  and  can  be  disregarded. 

t  A.  J.  Moses,  Characters  of  Crystals,  pp.  98,  99. 

\  The  terms  axes  of  elasticity  are  commonly  used  for  these  principal  vibration  direc- 
tions in  text-books  on  petrography. 

§  Instead  of  w  and  e,  for  conveniences  in  tables,  etc.,  a  and  y  are  used,  denoting  the 
indices  of  refraction  of  the  rays  traversing  the  crystal  with  greatest  and  least  velocity 
respectively,  without  regard  as  to  which  is  the  O  ox  E  ray.  A  good  reason  for  this 
convention  is  that  the  symbol  ( y  —  a )  is  used  to  express  in  decimals  the  relative  strength 
of  the  double  refraction  of  a  crystal,  whether  uniaxial  or  biaxial,  y  is  always  greater 
Ihan  a. 


4  INTRODUCTORY   OPTICS. 

To  this  group  belong  all  Tetragonal  and  Hcxagotial  crystals. 

(2)  Biaxial,  or  those  in  which  the  optical  characters  are  no 
longer  symmetrical  to  an  optic  axis  but  to  three  planes  at  right 
angles  to  each  other  (for  monochromatic  light).  These  crystals 
have,  however,  (for  light  of  each  wave-length  and  for  each  temper- 
ature) tiuo  directions  parallel  to  which  there  is  a  single  value  only 
for  the  light  velocity  and  hence  no  double  refraction.  These  direc- 
tions are  called  "  optic  axes."  *  An  investigation  of  these  biaxial 
crystals  shows  that  of  all  the  rays  traversing  these  crystals  there 
are  three  rays  which  advance  with  maximum,  minimum  and  some 
intermediate  velocity.  The  vibration  directions  of  these  three  rays 
are  called  the  prijicipal  vibratioii  directions  and  are  at  right  angles 
to  each  other  (being  the  intersections  of  the  three  planes  above 
referred  to).  The  direction  of  ether  vibration  of  the  fastest  ray  is 
denoted  by  a,  of  the  slowest  ray  by  c,  and  of  the  ray  advancing 
with  intermediate  velocity  by  b.t  Each  of  the  three  planes,  con- 
taining two  principal  vibration  directions,  is  called  an  optical  prin- 
cipal section.  The  index  of  refraction  of  the  a  ray  is  denoted  by  a, 
of  the  (1  ray  by  ;5,  and  of  the  c  ray  by  }-. 

To  this  group  belong  all  crystals  in  the  Orthorhouibic,  Moiio- 
cliiiic  and  Triclinic  systems. 

In  the  OrthorJionibic  system,  the  principal  vibration  directions 
are  parallel  to  the  crystallographic  axes  ;  hence  all  pinacoidal  sec- 
tions contain  two  of  these  principal  vibration  directions.  In  all 
sections  parallel  to  the  three  crystallographic  axes  a,  b  and  l\  the 
vibration  directions  are  parallel  or  symmetrical  to  cleavage  cracks, 
crystal  edges,  etc. 

In  the  Monoclinic  system,  one  principal  vibration  direction  is 
parallel  to  the  ortho  axis  b,  the  other  two  principal  vibration  direc- 
tions are  in  the  plane  of  symmetry,  at  right  angles  to  b,  but  are 
not  parallel  with  either  the  vertical  axis  c  or  the  clino  axis  c'(.  In 
clino  pinacoid  (co  P  co",  010)  sections  the  principal  vabration  direc- 
tions will  make  definite  angles  with  crystallographic  lines,  sucli  as 
cleavages  or  crystal  outlines.  These  angles  are  called  extinction 
angles.     They  will  vary,  in   this   system,    with  reference    to    the 

*For  most  cases  in  observations  with  white  light  the  "optic  axes"  maybe  regarded 
as  approximately  fixed  in  position. 

"t"  Often  spoken  of  as  :  0.,  the  axis  of  maximum  ;  h,  the  axis  of  intermediate,  and  C, 
the  axis  of  minimum  elasticity. 


EFFECTS  OF  CRYSTALS  ON  TRANSMITTED  LIGHT.         5 

direction  of  the  c  axis  from  a  maximum  on  the  cHno  pinacoid 
(cc  Pec,  oio)  to  o°  on  the  ortho  pinacoid  (cc  P  oc,  loo),  when 
the  N'ibration  directions  of  the  two  doubly  refracted  rays  will  be 
parallel  and  at  right  angles  to  the  c  axis.  Hence  the  vibration 
directions  are  parallel  or  symmetrical  to  cleavages,  edges,  etc., 
o)ily  in  sections  parallel  to  the  ortho  axis  b\  but  in  all  other 
sections  are  unsymmetrical. 

In  the  Triclinic  system,  the  principal  vibration  directions  are  not 
parallel  to  the  crj-stallographic  axes,  and  there  is  no  definite  relation 
between  these  directions  and  the  crystallographic  axes  ;  hence  in 
all  possible  sections  there  will  be  extinction  angles. 

In  all  biaxial  crystals  the  two  optic  axes  are  inclined  to  each 
other,  making  what  is  called  the  axial  angle,  2  V,  the  apparent 
angle  measured  in  air  being  2E.  The  optic  axes  lie  in  the 
plane,  called  the  axial  plaiic,  which  contains  the  principal  vibration 
directions  a  and  c.  The  axial  angles  are  bisected  by  these  principal 
vibration  directions,  the  direction  bisecting  the  acute  angle  being 
called  the  acute  bisectrix,  Bx^^."^'  An  approximate  idea  of  the  value 
of  the  axial  angle  can  be  obtained  by  the  use  of  the  petrographical 
microscope,  as  described  later,  p.  45.  The  axial  angle  is  often  a 
convenient  distinction  between  such  minerals  as  muscovite  and 
biotite. 

The  axial  angle  will  vary  with  the  temperature  and  with  light  of 
different  wave-length  or  color,  and  this  variation  is  called  disper- 
sion of  the  optic  axes.  Dispersion  of  the  principal  vibration  direc- 
tions also  takes  place  in  monoclinic  and  triclinic  crystals,  but  can  be 
disregarded  in  most  investigations. 

In  closing  it  is  very  important  to  remember  that  any  section  of 
an  anisotropic  crystal  (not  at  right  angles  to  an  optic  axis)  will 
always  transmit  two  rays  of  light  with  different  \'elocities  and  with 
vibration  directions  in  planes  at  right  angles  to  each  other.  Iso- 
metric cr}-stals,  of  course,  produce  no  double  refraction  of  light. 

*The  direction  bisecting  the  obtuse  angle  between  the  axes  is  called  the  obtuse  bi- 
sectric,  Bxo. 


CHAPTER    II. 

The  Petrographical  ^Microscope.* 

The  pctrograpliical  microscope  is  essential!}'  an  ordinary  micro- 
scope t  with  the  following  important  additional  equipment.  It 
must  be  provided  with  :  i  °  a  polarizer  (a  piece  of  apparatus  for 
giving  polarized  light)  placed  below  the  stage;  2°  an  analyzer  (a 
piece  of  apparatus  for  analyzing  the  rays  of  light  after  they  have 
passed  through  the  polarizer  and  transparent  section)  placed  be- 
tween the  objective  and  the  eye;  3°  a  stage  rotating  about  an 
axis  which  is  the  line  of  sight  of  the  microscope.  A  convenient 
type  of  microscope  is  that  made  by  Seibert  of  Wetzlar  (Xo.  1 1  <■?), 
Fig.  2. 

The  reflector  a  is  usually  fitted  with  a  plane  mirror  on  one  side, 
and  a  parabolic  mirror  on  the  other.  The  plane  mirror  should  be 
used  with  sunlight,  and  the  parabolic  mirror  with  artificial  light,  in 
order  to  make  the  rays  of  light  as  nearly  parallel  as  possible. 

The  polarizer  is  in  most  cases  a  nicol  prism,  set  in  a  suitable 
frame  b,  and  made  as  follows  : 

For  the  nicol  prism  "  a  cleavage  rhombohedron  of  calcite  (the 
variet}'  Iceland  spar  is  universally  used  in  consequence  of  its  trans- 
parency) is  obtained,  having  four  large  and  two  small  rhombo- 
hedral  faces  opposite  each  other.  In  place  of  the  latter  planes, 
two  new  surfaces  are  cut,  making  angles  of  68°  (instead  of  71°) 
with  the  obtuse  vertical  edges  ;  these  then  form  the  terminal  faces 
of  the  prism.  In  addition  to  this,  the  prism  is  cut  through  in  the 
direction  HH',  Fig.  3,  the  parts  then  polished  and  cemented  to- 
gether again  with  Canada  balsam.  A  ray  of  light,  al?,  entering  the 
prism,  is  divided  into  two  rays  polarized  at  right  angles  to  each 
other.  One  of  these,  dc,  on  meeting  the  layer  of  balsam  (whose 
refractive  index  is  less  than  that  of  the  ray  dc),  suffers  total  reflec- 

*  For  a  short  historical  sketch  of  the  use  of  the  microscope  in  connection  with  Pet- 
rology, see  G.  H.  William's  pamphlet.  Modern  Petrography  (^Monographs  of  Educa- 
tion), Boston,  1886. 

f  For  description  of  the  ordinary  microscope,  eye-pieces,  objectives,  magnification, 
etc.,  see  Manipulation  of  the  Microscope,  by  Ed.  Bausch. 

7 


8  THE  PETROGRAPIIICAL  MICROSCOPE. 


Fu;.    2. 


THE  PETROGRAPHICAL  MICROSCOPE.  9 

tion,  and  is  deflected  against  the  blackened  sides  of  the  prism  and 
extinguished.      The  other,  bd,  passes  through  and  emerges  at  e,  a 
completely  polarized  ray  of  light,  that  is,  a  ray  with  vibrations  in 
one  direction  only,  and  that  the  direction  of  the 
shorter  diagonal  of  the  prism."  *     The  vertical 
plane    through    the    shorter   diagonal    may   be 
called  the  plane  of  vibration  f  of  the  nicol. 

The  polarizer  must  be  below  the  stage  c,  and 
is  generally  adjusted  so  as  to  have  its  plane  of 
vibration  parallel  to  the  N.  and  S.  cross-wire 
in  the  eye-piece  o.  It  is  important  to  know  the 
direction  of  the  plane  of  vibration  of  the  polari- 
zer or  lower  nicol  ;  as  we  can  then  determine, 
when  absorption  of  light  occurs  in  a  mineral, 
the  direction  in  this  mineral  parallel  to  which 
the  absorbed  rays  are  vibrating.  The  polarizer 
slides  in  an  outer  shell  or  frame,  and,  by  means 
of  a  lever  d,  can  easily  be  raised  or  lowered. 

A  convenient  test  for  the  location  of  this 
plane  of  vibration  of  the  polarizer  is  as  follows  : 
Make  use  of  a  section  of  biotite,  cut  at  right 
angles  to  the  basal  plane,  hence  showing  the 
basal  clea\'age  cracks.  Biotite  has  the  propert}' 
of  absorbing  to  a  marked  extent  the  light  vibrating  parallel  to 
these  cleavage  cracks.  Rotate  such  a  section  on  the  stage  of  the 
microscope  until  the  position  of  maximum  darkness  is  reached  and 
when  such  is  the  case  the  plane  of  vibration  of  the  polarizer  must 
be  parallel  to  these  cleavage  cracks. 

On  the  top  of  the  nicol  is  placed  the  condensing  lens  for  getting 
convergent  light,  and  the  adjustments  are  so  arranged  that  when 
the  nicol  is  up  as  far  as  it  will  go,  the  condensing  lens  |  is  brought 
almost  in  contact  with  the  lower  surface  of  the  transparent  section 
resting  on  the  stage. 

The  rotating  circular  stage,  e,  is  supported  on  a  suitable  frame  c, 


Fig.  3. 


*Text  Book  of  Mineralogy,  by  E.  S.  Dana,  1898  Ed.,  p.  176. 

I  It  is  convenient  to  assume  tliat  the  vibrations  of  the  polarized  light  are  taking  place 
in  this  plane,  called  the  "plane  of  vibration,"  but  all  the  phenomena  caused  by  polar- 
ized light  cculd  be  also  explained  on  the  assumption  that  the  vibrations  were  taking 
place  at  right  angles  to  this  plane. 

X  This  condensing  lens  must  be  removed  when  very  low  power  objectives  are  used. 


lO  THh:  PETROGRAPHICAL  MICROSCOPE. 

and  arranged  so  that  its  axis  of  rotation  coincides  with  the  line  of 
sight  of  the  microscope.  The  stage  is  graduated,  and,  by  means 
of  an  index  fixed  to  the  frame,  the  angular  rotation  can  always  be 
obtained.  It  is  also  provided  with  two  adjusting  screws*/,  by 
means  of  which  the  axis  of  rotation  can  be  accurately  centered. 

The  method  of  centering  is  as  follows  :  Bring  some  prominent 
mark  in  the  section  exactly  in  coincidence  with  the  intersection  oi 
the  cross-wires  in  the  eye-piece.  Rotate  the  stage  i8o°,  and  cor- 
rect one  half  the  error  by  means  of  the  centering  screws,  and  the 
other  half  by  moving  the  section  on  the  stage.  Check  the  result 
by  rotating  the  stage  i8o°  again,  and  if  necessary  make  the  cor- 
rections in  the  same  way  until  the  adjustment  is  satisfactory. 

The  objcctivc'\  g  screws  into  the  collar  //,  which  has  a  slot  k,  in 
the  upper  portion,  for  the  introduction  of  a  sensitive  color  plate, 
a  ^  undulation  mica  plate  or  a  quartz  wedge.  % 

The  slot  k,  is  usually  so  arranged  that,  when  the  sensitive  color 
plates  are  introduced,  the  axes  of  these  plates  will  make  an  angle 
of  45°  with  the  planes  of  vibration  of  the  crossed  nicols,  and  the 
interference  color  produced  will  thus  be  at  its  maximum  intensity. 

A  revolving  >iosc-picce  is  sometimes  used  which  can  be  at- 
tached to  the  collar  h  and  arranged  to  carry  two  or  three  objec- 
tives, which  can  thus  be  very  quickly  brought  into  position  for 
use.  This  is  convenient  in  passing  rapidly  from  observations  with 
parallel  light  to  observations  with  convergent  light,  which  must  be 
made  with  a  high  power  objective.  The  difficulty  is  that  recenter- 
ing  is  generally  required.  The  modern  microscopes  are  provided 
with  a  clip  for  holding  the  objectives,  instead  of  the  screw-thread 
collar  h. 

The  analyser  §  or  upper  nicol  is  contained  in  the  frame  /,  which 


*Some  microscopes  are  provided  with  adjusting  screws  bearing  on  the  frame  holding 
the  objective,  which  can  then  be  accurately  centered  to  the  axis  of  rotation  of  the  stage. 

f  In  the  Seibert  microscope  use  objective  No.  00  for  the  first  general  study  of  a  rock 
section,  No.  II  for  general  use  and  No.  V  for  observations  with  convergent  light.  In 
the  Fuess  microscope  use  objective  No.  4  for  general  use  and  No.  7  for  convergent  light 
tests.  In  the  case  of  an  English  microscope  a  \''  to  f''  objective  is  used  for  general 
purpose  and  a  Y^  to  Y^  for  observations  with  convergent  light. 

X  See  pp.  32  and  :i2>- 

§  In  some  microscopes  the  analyzer  is  in  the  form  of  a  "cap"  nicol,  arranged  to  be 
fitted  over  the  top  of  the  eye-piece,  and  not  introduced  in  the  microscope  tube  as  shown 
here.  This  form  is  not  so  convenient,  as  the  "cap"  nicol  must  be  set  by  hand  every 
time  it  is  desired  to  make  observations  with  crossed  nicols.  But  at  the  same  time  it 
avoids  any  possible  refocusing  which  may  be  necessary  when  the  other  type  of  analyzer 
is  introduced  in  the  tube. 


THE  PETROGRAPHICAL  MICROSCOPE.  1 1 

is  arranged  so  as  to  slide  in  and  out  of  the  tube  of  the  micro- 
scope. 

The  plane  of  vibration  of  the  analyzer  is  fixed  by  the  instrument 
maker  so  as  to  be  at  right  angles  to  the  plane  of  vibration  of  the 
polarizer,  hence  in  the  Seibert  microscope  parallel  to  the  E.  and 
W.  cross-wire  in  the  eye-piece.  Consequently  when  the  frame  / 
is  pushed  into  the  tube,  the  analyzer  is  introduced  in  the  line  of 
sight  between  the  objectiv^e  and  the  observer's  eye,  with  its  plane 
of  vibration  at  right  angles  to  the  plane  of  vibration  of  the  polarizer  ; 
that  is  the  nicols  are  crossed.  When  the  nicols  are  crossed,  if  they 
are  properly  adjusted,  no  light  can  pass  through  to  the  eye  and 
the  field  of  view  should  be  dark. 

The  eyc-piccc'^  o,  fits  into  the  top  of  the  tube,  and,  by  means  of 
a  little  projecting  piece  fitting  into  a  slot  in  the  frame,  can  always 
be  adjusted  so  as  to  have  its  cross- wires  parallel  to  the  planes  of 
vibration  of  the  two  nicols. 

Some  instruments  are  provided  with  an  additional  slot  in  the 
tube  between  the  analyzer  and  the  eye-piece  for  the  introduction 
of  a  Bertrand  lens,  which  is  used  to  magnify  the  interference  figures 
produced  by  con\'ergent  light. 

The  first  approximate  focusing  is  made  by  the  screw  ;;/,  and  the 
fine  adjustment  by  the  screw  ;/. 

In  focusing  always  start  with  the  objective  very  near  the  section, 
and  move  it  away  until  the  right  focus  is  obtained.  Never  move 
down  towards  the  section  in  obtaining  the  focus,  as  there  is  danger 
then  of  striking  the  objective  against  the  section. 

For  cleaning  the  microscope  xylol  can  be  used,  as  it  will  not 
injure  the  lacquer.  To  lubricate  any  of  the  parts  use  a  small 
quantity  of  soft  tallow,  good  clock  oil  or  paraffin  oil. 

*  In  the  Seibert  microscope  eye-piece  No.  0  is  used  for  most  purposes.  Other  eye- 
pieces, Xos.  I.  and  III.,  with  cross-wires,  are  used  for  different  degrees  of  magnification, 
and  one  eye-piece  No.  II.,  without  cross-wires,  is  provided  to  be  used  in  connection 
with  an  eye-piece  micrometer.  Eye-pieces  with  cross-wires  and  different  degrees  of 
magnification  are  also  provided  with  the  Enghsh  microscopes. 


CHAPTER    III. 

Investigation  of  Microscopic  and  Optical  Char- 
acters OF  Minerals. 

Characters  of  Opaque  Minerals,  observed  by  refected  light. 

The  minerals  which  remain  perfectly  opaque  in  thin  rock  sec- 
tions have  usually  metallic  lustre  and  are  very  minute  in  size,  as 
is  the  case  with  the  iron  ores. 

When  the  metallic  minerals  of  a  rock  specimen  are  distinctly 
seen  with  the  unaided  eye,  it  is  rarely  necessary  to  make  a  section 
for  their  determination,  as  this  can  more  easily  be  done  by  any  of 
the  well  known  blowpipe  methods. 

Form,  Lustre,  Color,  Cleavage,  etc.,  are  recognized  in  the  same 
way  as  in  the  case  of  macroscopic  specimens.  In  order  to  make  the 
observations  by  reflected  light  alone,  the  beam  of  light  from  the 
reflector-mirror  must  be  cut  off  by  holding  the  hand  over  the  mir- 
ror or  by  moving  the  mirror. 

Characters  of  Transparent  Minerals,  observed  by  transmitted 
light.  The  petrographical  microscope  for  these  observations  is 
supposed  to  be  in  the  condition  of  an  ordinary  microscope,  both 
nicols  being  out  of  the  field  and  a  strong  beam  of  light  coming  up 
through  the  transparent  section  from  the  reflector  below. 

The  characters  are  considered  in  the  order  in  which  they  would 
most  naturally  appear  to  the  observer  during  a  complete  investiga- 
tion with  the  microscope.* 

{a)  Form.f  Complete  crystals,  with  definite  crystalline  outline, 
/.  e.,  crystals  that  have  formed  when  conditions  were  favorable  for 
complete  development.  Such  crystals  are  often  called  Idiomorp/iic, 
see   Fig.   4.      Crystals   that  are   large  in   comparison  with    other 

*  With  the  Seibert  microscope.  Fig.  2,  the  No.  0  eye-piece  and  the  No.  II.  objec- 
tive will  prove  most  satisfactory  for  the  following  tests.  With  the  Fuess  microscope 
use  No.  4  objective.  With  an  English  microscope  use  an  ordinary  eye-piece  and  a  i''^ 
or  j^'^  objective. 

f  Twins  may  be  recognized  just  as  in  macroscopic  specimens,  and  zonal  structure 
noticed  if  the  zones  differ  in  color.  When  a  colorless  mineral  is  surrounded  by  other 
colorless  minerals,  of  about  the  same  index  of  refraction,  its  outline  is  often  best  brought 
out  by  observation  between  crossed  nicols. 

13 


14      LWESTIGATION  OF  CHARACTERS  OF  MINERALS. 

accompanying  crystals  may  be  called  PJienockrysts.  Of  course 
crystals,  which  are  not  wholly  contained  within  the  rock  section, 
will  only  show  the  outline  of  the  bounding  planes  cut  by  this  par- 
ticular section.      In  some  cases,  by  a  careful   study  of  the  outline 


Fig.  4.  —  Idiomorphic  augite  crystal  in  camptonite.  The  section  is  about  at  right 
angles  to  the  vertical  axis  c ,  and  shows  the  intersecting  cleavages  parallel  to  the  prism 
of  87°  Ob'.      Keene  Valley,  N.  Y. 

of  several  sections  of  the  same  mineral  and  by  a  measurement  of 
the  angles,  it  is  possible  to  determine  the  common  "  crystallo- 
graphic  forms"  of  the  mineral.  Very  misleading  outlines  may, 
however,  be  observed  ;  as  for  example  the  triangular  outline  of  a 


Fig.  5.  —  Allotriomorphic  quartz  (/,  showing  no  "relief,"  plagioclase  /  and  decom- 
posed feldspar  o  in  granite.     As  seen  with  crossed  nicols. 


FORM. 


15 


section  of  a  cube,  with  the  corner  truncated.  For  measuring  the 
angles  have  the  stage  accurately  centered,  then  bring  the  vertex 
of  the  angle  to  be  measured  to  the  intersection  of  the  cross-wires 
in  the  eye-piece.  Bring  one  of  the  sides  in  coicidence  with  one 
of  the  cross-wires.  Note  the  reading  of  the  graduated  circle  and 
rotate  the  section  until  the  other  side  is  in  coincidence  with  the 
same  wire.  Take  the  reading  again  and  the  difference  will  be  the 
angle  required. 

Incomplete  Crystals,  without  definite  crystalline  outline,  whose 
bounding  surfaces  are  more  or  less  determined  by  those  of  ad- 
jacent crystals.  Such  crj-stals  are  frequently  called  Allotrio- 
niorphic,^'    see   Fig.    5.      This  allotriomorphic  form    must  not  be 


Fig.  6.  — Corroded  sanidine  crystal  in  perlite. 

confounded  with  the  worn  or  rounded  boundaries  of  the  com- 
ponent grains  in  clastic  rocks. 

Corroded  Cr)'stals,'\  whose  fretted  outline  is  undoubtedly  due  to 
corrosive  action  of  the  magma,  see  Fig.  6. 

Broken  or  Strained  Crystals.  In  some  cases  what  were  formerly 
larger  individuals  have  been  broken  or  shattered  by  dynamic  ac- 
tion into  much  smaller  fragments,  see  Fig.  7,  showing  crushed 
rim  of  fragments  surrounding  feldspar  "auge."      In  other  cases  an 

*  Xenomorphic  is  synonymous  with  allotriomorphic,  over  which  it  has  priority 
(Rohrbach).  The  term  "Anhedron,"  meaning  without  planes,  has  been  suggested 
\>y  L.  V.  Pirsson  to  describe  in  rocks  the  crystal  fragments  which  have  no  plane  faces, 
as,  for  example,  the  augites  of  augitic  rocks.  Science,  Jan.  lo,  1896,  p.  49. 

f  Partial  resorption  and  recrystallization  may  produce  a  border  of  secondary  min- 
-erals,  surrounding  the  original  crystal. 


1 6       LWESTIGATION  OF  CHARACTERS  OF  MINERALS. 

actual  bending  or  distortion  of  the  crystal  has  taken  place,  and 
again  sometimes  the  effect  of  mechanical  stress  is  only  shown  by 
the  so  called  "wavy"  extinction.      See  p.  30  and  Fig.  7,  showing 


Fig.  7. — Orthoclase  "auge"  (Carlsbad  twin),  showing  bending  and  "wavy" 
extinction,  surrounded  by  crushed  rim  of  mineral  fragments.  As  seen  with  crossed 
nicols.      "  Augen-gneiss,"  Bedford,  N.  Y.  —  B.   13. 

a  Carlsbad  twin   of  feldspar  that  has  been  bent  and  at  the  same 
time  shows  marked  "wavy"  extinction. 

Crystallites,  in  general  those  incipient  forms  of  crystals,  which 
have  not  yet  reached  a  stage  of  development  sufficient  to  show 


Pig.  8.  — Crystallites  and  .Microlites  a.     Skeleton  Crystals  b. 

double  refraction,   .see  Fig.  8,  a.      Quite  a  number  of  names  are 
used  to  describe  the  different  forms  that  occur. 

Microlites,  more  or  less  completely  defined  microscopic  crystals, 


IXDEX  OF  REFRACT/ON. 


17 


which  usually  show  double  refraction,  but  cannot  be  always 
specifically  determined,  see  Fig.  8,  a. 

Skeleton  Crystals  or  cr}-stallizations  which  have  not  produced 
entire  and  complete  individuals,  see  Fig.  8,  b. 

{b)  Color.  It  must  be  remembered  that  the  colors  observed 
are  always  due  to  transmitt^'^d  light  and  may  be  called  "  ahsoi'ption 
tints r  Minerals  which  in  hand  specimens  are  opaque,  are  often 
colored  in  sections  ;  and  minerals  which  are  commonly  colored 
may  appear  colorless  in  sections.  At  times  color  may  be  given  to 
a  section  simply  by  the  presence  of  a  great  number  of  minute 
inclusions. 

(r)  Index  of  Refraction.  //  =  sin  //sin  /-.  This  can  be  ap- 
proximately determined  by  the  appearance  of  the  surface  and  out- 
line of  a  mineral,  that  is  by  its  relief.      The  descriptive  terms  are 


Fig.  9.  — Olivine  crystal  in  basalt  showing  "high  relief"  and  cleavage. 

of  course  relative,  but  in  common  practice  minerals  are  said  to 
have  niediuui  refraction  when  the  refractive  index  is  between  that 
of  balsam  (1.54)  and  that  of  calcite  (1.60),  -sV;^?;/!^  refraction  when 
higher  than  1.60  and  zccak  refraction  when  lower  than  1.54.  In 
the  case  of  uniaxial  and  biaxial  minerals  the  mean  value  of  the 
indices  of  refraction  is  used. 

A  mineral  which  has  strong  refraction  appears  to  have  high 
relief,  i.  e.,  distinct  dark  contours  and  a  rough  or  "  shagreened  " 
surface,*  which  has  bright  illumination  and  appears  to  stand  out 


*  The  surfaces  of  all  minerals  in  sections  are  more  or  less  rough,  but  this  roughness 
is  only  made  visible  when  there  is  a  marked  difference  between  the  indices  of  refraction 
2 


I  8       /WESTIG  AT/ON  OF  CHARACTERS  OF  MINERALS. 

above  the  surfaces  of  the  surrounding  minerals  with  weaker  refrac- 
tion, see  Fig.  9. 

A  mineral  with  uicdinin  refraction  does  not  show  any  relief, 
hence  has  a  smooth  surface  and  no  dark  contours,  see  Fig.  5. 

A  mineral  with  iceak  refraction  appears  to  have  also  a  rough 
surface,  but  not  so  marked  as  in  the  case  of  a  mineral  with  ver>' 
strong  refraction,  on  account  of  the  smaller  contrast  in  indices 
between  the  mineral  and  balsam. 

The  practical  way  of  testing  for  the  approximate  index  of  refrac- 
tion or  the  relief  o{  a  mineral  is  to  make  use  of  the  lens  for  conver- 
gent light,  which  is  placed  on  top  of  the  lower  nicol  immediately 
below^  the  section. 

By  lowering"^  the  lens  the  character  of  the  relief  of  a  mineral 
section  is  made  very  apparent. 

In  order  to  become  familiar  with  the  way  in  which  the  "  relief" 
or  appearance  of  the  surface  indicates  the  strength  of  the  refractive 
index,  the  student  may  use  a  long  glass  slide  on  which  are  em- 
bedded in  balsam  (1.54)  small  fragments  of  different  minerals,  for 
example:  Sodalite  (1.483),  orthoclase  (1.523),  quartz  (1.547), 
topaz  (1.622),  hornblende  (1.642),  augite  (1.7 15),  epidote  (1.75  i), 
zircon  (i-95)  and  rutile  (2.712). 

Determinations  of  refractive  indices  in  sections  by  the  methods 
of  Chaulnes,t  Sorby,|  or  total  reflection  are  accompanied  by 
many  difficulties  and  may  fail  to  give  satisfactory  results.  The 
Becke  method,   however,   often  furnishes  a  convenient  means    of 


of  the  minerals,  and  the  index  of  refraction  of  the  balsam  in  which  the  minerals  are 
embedded.  The  index  of  refraction  of  balsam  is  about  1.54,  so  it  is  only  when  the 
mineral  has  a  high  index  of  refraction  that  its  surface  appears  rough.  When  internal 
structure  is  to  be  studied,  the  crystal  should  be  surrounded  by  a  fluid  of  nearly  the  same 
index  of  refraction  as  that  of  the  crystal,  and  wlren  the  exterior  of  the  crystal  is  to  be 
studied  then  a  fluid  should  be  used  with  a  very  different  index  of  refraction. 

*  The  .Seibert  microscope  has  a  very  convenient  and  quick  lowering  adjustment,  by 
means  of  the  lever  d,  for  making  this  test.  For  convenience  the  lower  nicol  and 
condensing  lens  are  generally  left  in  place  below  the  stage  of  the  microscope,  as 
polarized  light  serves  as  well  for  these  investigations  as  ordinary  light.  An  additional 
advantage  in  this  arrangement  is  that  the  condensing  lens  is  always  ready  for  the 
"  relief"  test  and  the  lower  nicol  for  the  pleochroism  test  ;  but  it  must  be  remembered 
that  the  polarizer  or  lower  nicol  cuts  out  one  half  of  the  light,  which  comes  to  it  from 
the  reflector,  and  this  loss  is  important  when  high  power  objectives  are  to  be  used. 
When  very  low  power  objectives  are  used  the  condensing  lens  must  be  removed. 

t  Mem.  de  V  Acad.,  Paris,  1767-68. 

\  Min.  Mag.,  Vol.  1.,  p.  193  ;   Vol.  II.,  p.   i. 


INDEX   OF  REFRACTION.  1 9 

determining  the  relative  values  of  the  refractive  indices  of  adjoin- 
ing minerals  or  of  minerals  embedded  in  balsam.  This  is  of 
especial  service  when  one  of  the  minerals  is  known  and  hence  its 
refractive  index. 

Becke  Method.* 

Suppose  two  adjoining  minerals,  in  a  thin  rock  section,  to  be 
singly  refracting  and  to  have  their  plane  of  contact  vertical,  /.  e., 
parallel  to  the  optic  axis  of  the  microscope. 

Let  A  and  C,  Fig.  lO,  be  two  such  sections  with  the  plane  of  con- 
tact 00'  vertical,  A  having  a  lower  refractive  index  than  C,  and 
consider  only  the  direction  of  the  rays  within  the  section,  neglect- 
ing the  refractive  effect  of  the  air,  glass  and  balsam. 

Let  a  cone  of  light  rays  GBI  be  concentrated  at  B.  The  rays 
O'  G  on  meeting  the  plane  of  contact  00'  will  be  somewhat  con- 
centrated and  deflected  by  the  higher  refractive  index  of  C  and  will 
continue  as  the  cone  EF.  Some  rays  as  0' H  will,  on  meeting  the 
contact  plane,  be  totally  reflected  and  will  continue  as  the  cone 
OE,  while  the  rest  of  the  rays  HI  will  be  dispersed  and  deflected 
by  the  weaker  refractiv^e  index  of  A,  continuing  as  the  cone  OD. 
Hence,  more  light  rays  will  emerge  on  the  side  of  the  contact 
plane  where  the  substance  of  higher  refractive  index  lies,  and 
there  will  be  a  concentration  of  illumination  on  this  side  producing 
the  so-called  "bright  line." 

In  practically  making  the  test  with  a  petrographical  microscope 
remove  the  polarizer,  analyzer  and  condensing  lens,t  and  intro- 
duce below  the  section  a  small  light-stop,  to  reduce  in  size  the 
cone  of  incident  light.  |  The  adapter,  holding  this  light-stop, 
should  be  adjustable  so  that  the  stop  can  be  lowered  sufficiently 
to  produce  the  best  results.  The  smaller  the  contrast  between 
the  indices  of  refraction  the  smaller  the  incident  cone  of  light 
should  be,  as  best  results  are  obtained  when  this  cone  is  little 
larger  than  twice  O' H,  Fig.  lo,  all  the  O' H  rays  being  then  totally 

*  Sitzungsberichte  der  k.  k.  Akad.  der  Wiss.,  Wien,  1893,  I.  Abt.,  p.  358.  Trans- 
lation by  L.  McI.  Luquer  in  School  of  Mines  Quarterly,  Vol.  XXIII.,  Jan.,  1902, 
No.  2,  p.  127.     Review  by  Viola  in  J/m.  Pet.  Mitt.,  Vol.  14,  p.  554. 

I  Most  of  the  petrographical  microscopes  carry  over  the  polarizer  a  convex  lens  the 
effect  of  which  is  to  widen  the  illuminating  cone  and  hence  make  less  visible  this  phe- 
nomenon. 

jIn  Seibert  student  microscope,  No.  11  a,  use  next  to  smallest  light-stop.  Some  of 
the  Fuess  microscopes  are  supplied  with  an  iris-blende  for  limiting  the  cone  of  light. 


20      INVESTIGATION  OF  CHARACTERS  OF  MINERALS. 

reflected.  The  more  of  the  HI  rays  that  pass  through,  the 
brighter  will  be  the  OD  cone  and  the  less  sharp  the  contrast  in 
illumination.  A  high-power  objective*  should  be  used,  as  those 
with  small  aperture  and  great  focal  length  do  not  give  good  re- 
sults. Focus  on  the  dividing  plane  and  adjust  until  equal  illumi- 
nation is  observed  on  both  sides  and  the  trace  of  the  plane  re- 
sembles a  fine  thread,  the  focal  plane  being  at  MN.  -  Then  raise 
the  objective  slightly,  thus  moving  the  focal  plane  to  ST,  when  a 
bright  /i)ie  or  band  will  appear  on  the  side  of  the  stronger  refract- 
ing substance,  the  width  of  the  line  depending  on  the  contrast  be- 


tween ;/  and  ;/',  becoming  narrower  as  ;/  and  //  approach  each 
other  in  value.  On  raising  the  objective  still  further  the  line 
broadens  and  finally  disappears.  If  the  objective  is  lowered  in- 
stead of  raised  the  reverse  phenomenon  will  take  place,  the  bright 
line  appearing  on  the  side  of  the  weaker  refracting  substance,  t 

The  appearance  of  equal  illumination  and  the  absence  of  bright 
lines  on  either  side  when  the  focal  plane  is  at  MN  will  be  only 
strictly  true  in  the  case  of  isotropic  minerals  or  basal  sections  of 
uniaxial  minerals.  With  doubly  refracting  minerals,  each  set  of 
doubly  refracted  rays  would  suffer  total  reflection  under  somewhat 

*In  Seibert  microscope  use  No.  V.,  not  sufficiently  marked  results  being  obtained 
with  No.  II. 

"{"See  Viola's  diagram,  Minn.  Pet.  Mitt.,  Vol.  XIV.,  p.  556. 


INDEX   OF  REFRACT/OX. 


21 


different  conditions  and  there  would  be  no  one  point  B  where  no 
bright  Hnes  would  appear.  However,  if  the  cone  of  incident  light 
is  small  enough  in  diameter,  the  test  will  be  practically  obtained  as 
described.  The  Becke  test  can  often  be  made  with  a  medium 
power  objective  *  without  removing  the  polarizer  and  condensing 
lens,  provided  the  lens  be  lowered  sufficiently  to  get  rid  of  the 
strongly  divergent  rays. 

The  thinner  the  section  the  more  distinct  will  be  the  phenom- 
enon. The  contact  plane  must  be  clear  and  not  coated  with 
opaque  decomposition  products. 

When  the  plane  of  contact  00'  deviates  considerably  from 
parallelism  with  the  optic  axis  of  the  microscope  a  disturbance  of 
the  phenomenon  may  be  expected  and  no  satisfactory  results  be 
obtained.  In  general  under  these  conditions  the  "  bright  line," 
both  on  raising  and  lowering  the  objective,  will  remain  on  the  side 


Fig.  II.  — Biotite,  showing  perfect  cleavage,  in  rhyolite.  A  fragment  of  a  sanidine 
crystal  is  seen  at  j-. 

of  the  overlapping  substance,  without  regard  to  the  relative  values 
of  the  indices. 

The  refractive  index  of  the  cementing  material  should  not  be 
very  much  lower  than  that  of  either  of  the  thin  sections,  as  the  re- 
sult would  then  be  to  disperse  the  emerging  rays  too  much  and 
dim  the  effect.  In  the  case  of  distinguishing  between  minerals  of 
high  refractive  power,  such  as  augite  and  garnet,  methyliodide  is 
recommended  instead  of  balsam. 

*Seibert,  No.  II.     Fuess,  No.  4. 


2  2       INVESTIGATION  OF  CHARACTERS  OF  MINERALS. 


In  the  case  of  doubly  refractinfy  crystals,  the  lower  nicol  (polar- 
izer) must  be  retained,  if  it  is  desired  to  obtain  the  relative  refrac- 
tive index  of  one  of  the  two  rays,  either  with  respect  to  the  balsam 
or  to  a  ray  in  an  adjoining  ciystal  with  a  parallel  vibration  direc- 
tion. For  determining  vibration  directions  of  rays  and  the  faster 
and  slower  rays  in  two  adjoining  sections  see  later,  pp.  30  and  32. 

When  isolated,  transparent  fragments  of  the  mineral  can  be  ob- 
tained "  the  index  of  refraction  may  be  approximately  determined 
by  immersing  small  fragments  of  the  mineral  in  a  series  of  liquids 
of  known  refractive  indices.*  If  the  mineral  and  the  liquid  have 
nearly  the  same  indices  the  light  will  go  through  without  notice- 
able bending  and  the  outline  and  roughness  will  be  invisible,  but 
if  they  differ  materially  the  roughness  will  be  distinctly  visible. 

F.  Krantz,  of  Bonn,  furnishest  a  series  of  21  liquids  in  glass-stop- 
pered bottles,  the  indices  of  the  liquids  ranging  from  1.447  to  1.83." 


Fig.  12. — Augite  a,  showing  good  cleavage,  and  plagioclase  /  in  diabase.  The 
plagioclase  shows  "  polysynthetic  "  twining  between  crossed  nicols. 

In  addition  to  noting  this  surface  appearance,  the  Becke  "  bright 
line"  test  should  also  be  made. 

id)  Cleavage,."}]  which  appears  as  more  or  less  distinct  and 
regular  lines  or  cracks,  see  Figs.  1 1  and  12. 

These  cleavage  cracks  may  be  parallel  or  intersect,  depending 

*  The  indices  of  a  few  convenient  liquids  are  :  water,  I.34  ;  alcohol,  I.36  ;  glycerine, 
1. 41  ;  olive  oil,  1. 47  ;  nut  oil,  I.50  ;  clove  oil,  1.54;  aniseed  oil,  1.58;  almond  oil, 
1.60;  cassia  oil,  1.63;  monobromnapthalene,  1.65  ;  methylene  iodide,  1. 75. 

t  Price  12  marks. 

%  Crystals  that  have  two  good  cleavages  often  develop  so   that  the  direction  of  elon- 


CLEAVAGE. 


23 


on  the  position  of  the  section  relative  to  the  cleavage  planes  of  the 
crystal. 


Fig.  13.  — Garnet  in  mica  schist,  showing  fracture  and   "high  relief."      Franconiaj, 
N.  Y. 

Cleavage  is  sometimes  best  observed  by  slightl}'  lowering  the 
condensing  lens  under  the  section. 

When   sections   show   intersecting'  cleavage  cracks   it    is   often 


Fig.  14. — Apatite  in  feldspar  a.     Garnets  in  quartz,  Branchville,  Ct.,  b.      Liquid 
inclusions  of  CO,,  some  showing  gas  bubbles,  in  quartz  <•. 

possible  to  recognize  the  mineral  by  its  known  cleavage  angle,  as 
in  the  case  of  amphibole  and  pyroxene. 

gation  is  parallel  to  the  intersection  of  the  two  cleavages,  while  in  the  case  of  crystals 
with  one  good  cleavage  the  tendency  seems  to  be  towards  a  tabular  habit  parallel  to  the 
cleavage. 


24       LWESTIGATION  OF  CHARACTERS  OF  MINERALS. 

{e)  Fracture,  wliich  appears  as  irregular  and  non-parallel  cracks, 
see  Fig.  13. 

(/)  Inclusions,  which  may  be  solid  (either  distinct  crystals  or 
-glass),  fiiiid  ox  gas,  see  Fig.  14.  These  inclusions  are  distinguished 
by  the  fact  that  the  solid  inclusions  generally  have  sharp  contours, 
the  fluid  inclusions  distinct  dark  borders,  and  the  gas  inclusions 
broad  dark  boarders  more  distinctly  marked  than  those  of  the  fluid 
inclusions.  The  fluid  inclusions  often  contain  a  bubble  of  gas. 
The  inclusions  may  have  a  definite  or  indefinite  position  in  the 
crystal  in  which  they  occur,  and  can  sometimes  be  more  distinctly 
seen  by  using  convergent  light  or  a  "  spot "  lens. 


Fig.    15.  —  Enstatite  showing  "  Schiller"  Structure. 

{g)  Schiller  Structure.  "  Is  that  in  which  cavities  of  definite 
form  and  orientation  ('  negative  crystals  ')  are  developed  along  cer- 
tain planes  and  filled  or  partially  filled  by  material  dissolved  out 
of  the  enclosing  crystal,"*  see  Fig.  15. 

Characters  Observed  by  Polarized  Transmitted  Light. 
The  polarized  light  is  obtained  by  passing  the  beam  of  light 
from  the  reflector  through  the  polarizer  or  lower  nicol,  f  which 

*Harker's  Petrology  for  Students,  p.   306. 

t  The  lower  nicol  is  generally  so  adjusted  that  its  plane  of  vibration  is  parallel  to 
the  north  and  south  cross-wire  in  the  eye-piece.  This  adjustment  can  be  tested  by 
means  of  a  section  of  biotite,  showing  cleavage  cracks.  When  the  plane  of  vibration 
■of  the  polarizer  is  parallel  to  the  N.  and  S.  cross-wire  in  the  eye-piece,  the  biotite  sec- 
tion becomes  almost  dark  when  its  cleavage  cracks  are  parallel  to  the  same  cross-wire. 
The  upper  nicol,  or  analyzer,  must,  of  course,  be  removed  during  this  test.  This 
method  is  more  convenient  than  taking  the  nicol  out  of  its  frame,  in  order  to  ascertain 
nts  plane  of  vibration  (the  direction  of  its  shorter  diagonal). 


CHARACTERS  WITH  CROSSED  NICOLS.  25 

must  be  in  place  below  the  stage  of  the  microscope.      White  light 
is  supposed  to  be  used. 

Pleochroism,  that  property  which  all  anisotropic  minerals  have, 
to  a  greater  or  less  extent,  of  absorbing  certain  colored  rays  in 
certain  directions,  thereby  showing  different  colors  in  different 
directions  by  transmitted  light.  Uniaxial  minerals  are  dichroic 
showing  two  differences  in  color,  produced  by  the  ra}s  which 
vibrate  parallel  to  the  direction  of  the  vertical  axis  c  and  the  plane 
of  the  basal  axes.  Biaxial  crystals  are  trichroic  showing  theoret- 
ically three  differences  of  color  produced  by  rays  with  vibration 
directions  corresponding  very  nearly  to  those  of  the  three  principal 
vibration  directions.*  Any  given  section  of  a  biaxial  crj^'stal  will, 
of  course,  appear  only  dichroic. 

The  practical  way  of  testing  for  pleochroism  is  as  follows  :  Re- 
volve the  stage,  carrying  the  section,  when  a  change  in  the  color 
of  the  mineral  will  be  noticed,  if  it  is  pleochroic.  This  pleochroism 
may  appear  as  an  actual  change  in  color  or  simply  as  a  change  in 
the  shade  of  the  same  color.  At  times  it  may  be  so  weak  as 
hardly  to  be  noticed,  when  it  is  best  to  make  the  test  with  the 
condensing  lens  in  position  immediately  under  the  section,  or  by 
rotating  the  lower  nicol  instead  of  revolving  the  stage.  The  color 
of  the  light,  vibrating  parallel  to  certain  definite  crystallographic 
directions  in  minerals,  is  often  very  characteristic. 

In  some  cases  there  may  be  such  strong  absorption  of  one  of  the 
rays,  that,  when  the  vibration  direction  of  this  ray  is  over  the  plane 
of  vibration  of  the  polarizer,  practically  no  light  is  transmitted  and 
the  section  appears  dark.  Strong  absorption  is  characteristic  of 
certain  minerals,  such  as  biotite,  amphibole,  tourmaline  and  allanite 
and  takes  place  parallel  to  definite  directions  in  these  minerals. 

Pleochroism  may  often  be  noticed  with  ordinary  light  in  the 
case  of  hand  specimens. 

Characters  Observed  with  both  Polarizer  and  Analyzer  t  in  Po- 
sition, that  is  with  <'  Crossed  Nicols." 
When   the  nicols  are  accurately  crossed,   the   field   should   be 
quite  dark,  and  if  this  is  not  the  case  the  adjustments  must  be 

*  Although  the  "absorption  directions"  may  not  necessarily  coincide  with  the 
principal  vibration  directions  in  Monoclinic  and  Triclinic  crystals  ;  still  for  convenience 
the  absorption  colors  are  usually  given  for  the  light  rays  vibrating  parallel  to  these 
principal  vibration  directions. 

t  In  the  Fuess  and  Seibert  microscopes  the  analyzer  or  upper  nicol  is  so  fitted  that 


26       IXl'ESTIGATION  OF  CHARACTERS  OF  MINERALS. 

looked  to.  The  condensing  lens  should  be  removed  for  these 
tests  as  they  are  to  be  made  with  parallel  light,  but  as  a  matter  of 
convenience  instead  of  removing  the  condensing  lens,  the  polarizer 
with  the  lens  on  top  may  be  lowered,  when  the  results  will  be 
about  the  same  as  with  parallel  light.  W'hite  light  is  supposed  to 
be  used. 

Isotropic  Character.  Sections  of  isotropic  crystals  are  per- 
fectly dark  and  remain  so  during  a  complete  rotation  of  the  stage 
through  360°.  The  explanation  is  very  simple.  Tight  being 
transmitted  by  an  isotropic  crystal  in  all  directions  without  double 
refraction  ;  it  follows  that  the  light  from  the  polarizer,  after  having 
passed  through  the  section,  comes  to  the  analyzer  still  vibrating 
in  the  plane  of  vibration  of  the  polarizer.  Hence  it  is  entirely  cut 
out  by  the  analyzer. 

Amorphous  transparent  substances  act  in  the  same  way  and 
remain  dark  during  complete  rotation  of  the  stage. 

Optical  anomalies,  i.  e.,  apparent  double  refraction,  may  occur  in 
isometric  crystals  and  in  amorphous  substances  that  have  been 
subjected  to  strains. 

Anisotropic  Character.  Sections  of  anisotropic  crystals,  hav- 
ing the  property  of  double  refraction,  produce  in  general  some 
iiiterfereuce  or  polarization  color,  except  as  mentioned  later.  The 
popular  explanation  is  as  follows  : 

In  Fig.  16,  let  PP'  be  the  plane  of  vibration  of  the  polarizer, 
and  A' A  the  plane  of  vibration  of  the  analyzer.  All  the  light, 
after  it  has  passed  through  the  polarizer,  is  vibrating  parallel  to 
PP'  when  it  reaches  the  lower  side  of  the  transparent  crystal,  cdef, 
on  the  stage  of  the  microscope.  In  this  transparent  section  let 
ob  and  oa  be  the  two  directions  of  vibration,  i.  e.,  the  only  two 
directions  parallel  to  which  rays  of  light  can  vibrate  in  passing 
through  the  section. 

Let  om  represent  the  amplitude  of  vibration  of  a  ray  from  the 
polarizer.  When  this  ray  reaches  the  section  it  cannot  get  through 
it  vibrating  in  the  direction  oni,  but  is  doubly  refracted  and  of  the 
two  resulting  rays,  one  gets  through  vibrating  in  the  direction  ob 
and  the  other  vibrating  in  the  direction  oa.  From  ;;/  draw  per- 
pendiculars to  ob  and    oa.     Then  according    to    the    law  of  the 

it  slides  in  and  out  of  the  tube  of  the  microscope  with  its  plane  of  vibration  always  at 
right  angles  to  the  plane  of  vibration  of  the  polarizer  or  lower  nicol. 


AXISO  TR  OPIC  CHAR  A  CTER. 


27 


parallelogram  of  forces  ob  will  represent  the  amplitude  of  vibration 
of  the  ray  passing  through  the  crystal  vibrating  in  the  direction 
ob,  and  oa  will  represent  the  amplitude  of  vibration  of  the  ray 
passing  through   the  cr}'stal  vibrating   in   the   direction  oa.     We 


will  thus  have  two  rays  passing  through  the  cr}'stal,  polarized  at 
right  angles  to  each  other. 

Consider  now  the  general  question  of  the  transmission  of  the 
doubly  refracted  rays  through  a  plate.  Whatever  the  angle  of 
the  parallel  incident  rays,  each  ray  as  AB, 
Fig.  17,  is  resolved,  as  just  described,  into  two 
rays  BC  and  BD,  polarized  at  right  angles  to 
each  other  and  following  (usually)  different 
paths  in  the  plate.  On  emergence  these  fol- 
low parallel  paths.  Among  the  incident  rays 
there  are  rays  EG  and  FH,  such  that  one 
component  of  FH  will  emerge  at  D  with  the 
other  component  of  AB,  and  one  component 
of  AB  and  another  component  of  EG  will 
emerge  at  C.  Hence  from  every  point  of  the  upper  surface  of 
the  plate  there  will  emerge  two  rays  and  these  rays  will  ha\-e 
travelled     through    different    paths    in    the    plate    with     different 


28       IXVESriGATION  OF  CHARACTERS  OF  MINERALS. 

velocities  and  will  have  their  vibrations  at  right  angles  to  each 
other. 

When  these  doubly  refracted  rays  come  to  the  analyzer, 
whose  plane  of  vibration  is  A' A,  they  cannot  get  through 
vibrating  in  their  present  directions,  but  components  of  these 
rays  such  as  ot  and  os,  can  get  through  vibrating  parallel  to  the 
plane  A' A. 

Hence  we  have  two  series  of  rays  coming  to  the  eye,  polarized 
in  the  same  plane,  but  one  set  slightly  in  advance  ot  the  other. 
These  rays  will  "interfere"  and  produce  some  i)itcrferencc  or 
polarization  color.*  In  using  white  light  whenever  one  of  two 
light  rays  of  the  same  color  has  suffered  a  "retardation  "  of  just 
one  wave-length  (or  even  multiple  thereof)  the  color  will  be  ex- 
tinguished ;  and  when  the  "retardation"  is  one  half  w'ave-length 
(or  even  multiple  thereof)  the  color  will  be  intensified.  There- 
fore, some  tints  will  .be  extinguished  and  others  intensified,  the 
combination  resulting  in  the  production  of  some  definite  inter- 
ference color.  Of  course  in  the  case  of  monochromatic  light 
the  thickness  of  the  section  may  be  such  that  "  destructive  inter- 
ference "  takes  place  producing  no  color  (darkness).  The  inter- 
ference color  is  independent  of  the  proper  color  or  "  absorption 
tint"  of  the  crystal  section. 

Now  suppose  the  stage  to  be  rotated  until  the  section  cdef 
takes  the  position  c'd'c'f .  The  section  will  be  found  to  be  dark 
and  no  interference  color  will  be  seen.  This  is  due  to  the  fact  that 
the  directions  of  vibration  in  the  section  are  parallel  to  the  planes  of 
vibration  of  the  crossed  nicols,  consequently  the  light  passes 
through  the  section  still  vibrating  parallel  to  the  plane  PP'  of  the 
polarizer  and  is  all  cut  out  by  the  analyzer.  Darkness  will  occur 
every  90°  and  therefore  four  times  during  a  complete  rotation  of 
the  stage.  The  interference  color  is  also  observed  to  vary  in  inten- 
sity, but  not  in  color,  and  to  be  at  its  maximum  45°  from  the  posi- 
tions of  darkness. 

Sections  of  uniaxial  crystals  at  right  angles  to  the  optic  axis  act 
like  isotropic  substances  and  remain  dark  during  a  complete  rota- 
tion of  the  stage.  In  the  biaxial  crystals,  a  section  at  right  angles 
to  an  "  optic  axis  "  shows  illumination  or  a  color  tint,  due  to  "  inter- 

*  Moses'  Characters  of  Crystals,  p.  lo6.  Moses  and  Parsons'  Min.  Cyst,  and  B. 
P.  Analysis,  p.  163. 


INTERFERENCE  COLORS.  29 

nal  conical  refraction,"  which  does  not  change  as  the  stage  is 
rotated.  * 

The  only  way  to  find  out  whether  these  uniaxial  sections  are 
truly  isotropic,  or  simply  at  right  angles  to  an  optic  axis,  is  to 
test  them  with  convergent  light  as  described  later,  see  p.  38. 

Interference  Colors.  The  interference  color  shown  by  any 
mineral  section  depends  on  three  factors  :  i  °  the  strength  of  the 
double  refraction  (;-  —  a  =  the  difference  between  the  refractive 
indices  of  the  slowest  and  fastest  rays) ;  2°  the  position  of  the 
section  in  the  crystal ;   3  °  the  thickness  of  the  crystal  section. 

To  eliminate,  so  far  as  possible,  the  variation  due  to  the  optical 
orientation  of  the  section,  care  must  always  be  taken  to  obtain  the 
maximum  interference  color  given  by  the  different  sections  of  the 
same  mineral  in  the  rock  section.  Sections  giving  the  maximum 
color  are  those  parallel  to  the  c  axis  in  uniaxial  minerals  and  those 
parallel  to  the  axial  plane  in  biaxial  minerals.!  Hence  such  sec- 
tions in  convergent  light  never  show  the  emergence  of  an  optic 
axis  or  a  bisectrix  ;  and  furthermore,  other  clues,  such  as  crystal 
outlines,  cleavage,  pleochroism,  etc.,  may  help  to  indicate  the 
favorable  section. 

The  influence  of  the  thickness  of  the  section  is  also  important 
and  must  be  considered.  The  interference  color  will  rise  in  the 
scale  (be  higher  in  order)  as  the  section  becomes  thicker.  Methods 
of  obtaining  the  thickness  of  a  section  are  given  on  pp.  34  and  35. 
When  reference  is  made  to  definite  interference  colors  the  mineral 
section  is  supposed  to  have  a  thickness  of  0.03  mm.  and  to  be  such 
as  to  give  the  maximum  color  ;  and  all  mineral  sections  in  a  rock- 
section  are  considered  to  have  a  uniform  thickness. 

With  these  precautions  in  mind  the  interference  colors  indicate  the 
strength  of  the  double  refraction  as  follows  :  The  colors  of  minerals 
with  very  weak  double  refraction  vary  from  a  bluish-gray  to  a 
grayish-white.  As  the  strength  of  the  double  refraction  increases 
the  colors  become  the  very  intense  bright  tints  of  the  spectrum, 
yellow,  red,  blue,  green,  etc.,  called  the  first  and  second  order 
colors.  As  the  strength  of  the  double  refraction  still  increases  the 
colors  pass  through  the  tints  of  the  spectrum  in  sequence  (called 
orders),  becoming  paler  until  finally,  when  the  double  refraction  is 

*  Moses'  Characters  of  Crystals,  p.  136. 

f  These  sections  always  contain  the  principal  vibration  directions  a  and  c. 


30       LWESTIGATION  OF  CHARACTERS  OF  MINERALS. 

very  strong,  the  colors  become  the  neutral,  almost  colorless  tints 
of  the  higher  orders.  The  eye  must  be  trained  to  appreciate  the 
colors  of  different  orders,  and  the  student  is  advised  to  practice 
with  mineral  sections,  of  known  strength  of  double  refraction,  and 
compare  the  resulting  interference  colors  with  a  color  chart,*  or 
use  the  interference  color  diagram  at  the  end  of  the  book.  A  con- 
venient test  can  be  made  with  a  Vj^  undulation  mica  plate  to  dis- 
tinguish between  the  white  of  the  i  °  order  and  the  practically 
white  \er}^  high  order  tint.  Introduce  the  mica  plate  in  the  slot 
/',  Fig.  2,  and  the  effect  will  be  to  produce  a  marked  change  in 
the  color  of  the  i  °  order,  while  no  change  will  be  observed  in 
the  high  order  color. 

The  exact  order  of  the  color  can  be  determined  by  the  use  of  a 
quartz-wedge,  as  described  on  p.  34. 

Extinction  and  Extinction  Angles.  When  the  section,  see 
Fig.  16,  is  in  such  a  position  that  its  directions  of  vibration  are 
parallel  to  the  planes  of  vibration  of  the  nicols,  no  light  can  pass 
through  the  analyzer,  and  the  section  is  dark.  Hence  the  light  is 
extinguished  and  this  phenomenon  is  called  extinction. 

Extinction  is  said  to  be  parallel  or  symmetrical  when  the  direc- 
tions of  vibration  are  parallel  to  any  crystallographic  lines  or  direc- 
tions, or  bisect  the  angles  between  these  lines.  The  crystallo- 
graphic lines  or  directions  may  be  either  cleavages  or  the  similar 
boundaries  of  idiomorphic  crystals. 

This  kind  of  extinction  is  shown  by  all  sections  of  tetragonal  and 
hexagonal  crystals,  by  all  sections  parallel  to  the  crystal  axes  in 
orthorhombic  crystals  and  also  by  the  sections  parallel  to  the  b 
axis  in  monoclinic  crystals. 

When  the  extinction  is  not  s)'mmetrical  it  is  called  oblique,  and 
is  shown  by  all  sections  of  monoclinic  crystals  (except  those  par- 
allel to  the  b  axis)  and  triclinic  crystals. 

Extinction  which  does  not  take  place  over  the  whole  of  the  sec- 
tion of  a  single  crystal  at  the  same  moment,  but  passes  over  the 
section  like  a  dark  wave  or  shadow  is  said  to  be  "ti'^rj"  .•  and 
indicates  that  the  crystal  has  been  subjected  to  mechanical  forces 
producing  a  change  in  the  position  of  the  directions  of  vibration  in 
different  parts  of  the  crystal,  see  Fig.  7. 

*  A  chart  of  interference  colors  can  be  obtained  from  Baudry  et  Cie,  Paris,  and  is 
also  published  in  Les  Mineraiix  des  RocJies,  by  Levy  and  Lacroix,  and  in  Rosen- 
busch's  MikroskopiscJie  Pltysiog7-ap)iie. 


EXTLXCTIOX  AXD  EXTIXCTIOX  AXGLES.  3  I 

The  angle  between  a  direction  of  vibration  in  the  section,  and 
some  known  crystallographic  direction  (as  a  cleavage  or  crystal 
outline)  is  called  an  extinction  angle,  and  is  measured  in  the  follow- 
ing way  :  Find  the  positions  of  the  directions  of  vibration  in  the 
section,  which  must  be  parallel  to  the  cross-wires  when  extinction 
takes  place.  Note  the  reading  on  the  graduated  circle  of  the  stage, 
then  remove  the  upper  nicol,  in  order  to  get  a  more  distinct  view 
of  the  field,  and  rotate  the  stage  until  you  bring  some  known  cr)'s- 
tallographic  line  into  parallel  position  with  the  cross-wire  selected 
as  a  reference  line.  Take  the  reading  of  the  graduated  circle  again, 
and  the  difference  between  these  two  readings  will  be  the  extinction 
ano-le. 

o 

It  can  readily  be  seen  that  depending  on  which  way  the  section 
is  rotated,  either  a  small  or  a  large  extinction  angle  will  be  obtained, 
the  two  angles  being  complements  of  each  other.  The  extinction 
angle  generally  recorded  is  that  between  the  nearest  direction  of 
vibration  and  the  vertical  axis  c. 

In  monoclinic  minerals  the  maximum  value  of  the  extinction 
angle  (the  angle  of  real  v^alue  in  distinguishing  the  mineral)  can 
only  be  obtained  from  a  section  parallel  to  the  clinopinacoid  (  ccPa;  , 
Oio)  ;  but  in  practice  sufficiently  accurate  results  can  generally  be 
obtained  by  measuring  the  extinction  angles  of  all  sections  of  the 
same  mineral,  w^hich  seem  to  be  about  parallel  to  the  vertical  axis 
c,  and  then  taking  the  maximum  value  obtained.  Amphibole  and 
pyroxene  can  easily  be  distinguished  in  this  wa}'. 

Extinction  is  generally  tested  for  b}^  revolving  the  stage,  carry- 
ing the  section,  until  darkness  is  observed  ;  the  most  accurate  re- 
sults being  obtained  by  using  monochromatic  light.  There  are 
other  more  exact  methods,  *  called  Stauroscopic  methods,  for  locat- 
ing these  directions  of  vibration.  A  quartz  or  gypsum  test-plate, 
so  prepared  as  to  give  some  definite  interference  color,  as  red  of 
the  I  °  order,  may  be  introduced  between  the  two  nicols,  and  the 
section  revolved  until  this  color  is  exactly  matched.  This  perfect 
matching  of  color  is  only  possible  when  the  directions  of  vibration 
of  the  section  are  exactly  parallel  to  the  planes  of  vibration  of  the 
nicols,  as  otherwise  some  interference  color  would  be  produced  by 
the  section  and  the  true  color  of  the  test-plate  would  not  appear. 
The  most  favorable  condition  is  when  the  section  only  covers  a 

*  Iddings'  Rosenbusch,  p.  63. 


32       IXVESTIG AT/OX  OF  CHARACTERS  OF  MINERALS. 

part  of  the  field  of  view,  as  then  the  rest  of  the  field  shows  the  color 
of  the  test-plate  and  the  exact  matching  of  color  is  an  easy  matter. 
This  method  of  testing  can  be  employed  to  recognize  the  very 
weak  double  refraction  of  some  minerals,  whose  interference  colors 
are  such  dark  grays  as  not  to  be  noticed  without  this  test.  After 
the  test-plate  has  been  introduced,  some  slight  variation  in  the 
color  will  be  observed  when  the  stage  carrying  the  doubly  refract- 
ing section  is  rotated. 

Method  of  Testing  for  the  Vibration   Directions  of  the 

Faster  and  Slower  Rays,  a'  and  c',  in  a 

Mineral  Section. 

A  mica,  or  gypsum  plate  can  be  used  to  make  this  test,  the 
directions  of  vibration  of  the  faster  and  slower  rays  on  these  plates 
being  known  and  marked.* 

The  crystal  section  to  be  tested  is  placed  on  the  rotating  stage, 
between  crossed  nicols,  and  the  directions  of  vibration  of  these  two 
rays  determined  b}'  finding  the  directions  of  extinction.  The  sec- 
tion is  then  turned  45°,  when  the  interference  color  will  be  at  its 
maximum,  and  the  directions  of  vibration  will  make  angles  of  45° 
with  the  planes  of  vibration  of  the  nicols  and  the  cross-wires  in  the 
eye -piece. 

Now  introduce  t  the  test-plate,  between  the  section  and  the  an- 
alyzer, so  that  its  known  directions  of  vibration  also  make  angles 
of  45°  with  the  cross-wires  in  the  eye-piece. 

When  the  test-plate  is  introduced  a  new  interference  color  will 

*The  }l  undulation  mica  plate  consists  of  a  thin  cleavage  of  mica  on  which  is 
marked  c,  the  vibration  direction  of  the  slower  ray,  which  in  mica  is  the  line  joining 
the  "  optic  axes."  The  thickness  is  such  that  the  slower  ray  is  %  wave-length  behind 
the  faster  and  the  interference  color  is  a  bluish-gray.  The  gypsum  plate  is  a  thin 
cleavage  of  gypsum,  on  which  is  usually  marked  a,  the  vibration  direction  of  the  faster 
ray.  The  chosen  thickness  is  such  as  to  produce  the  red  interference  color  of  the  i° 
order. 

I  The  test-plates  are  generally  introduced  in  the  slot  /•,  in  a  microscope  of  the  Sei- 
bert  type,  or  if  a  cap-nicol  is  used  in  a  slot  below  this.  In  case  no  provision  is  made 
by  the  instrument  maker  for  these  test-plates,  the  regular  analyzer  is  left  out  of  the 
tube,  and  a  simple  nicol  prism  is  used  as  an  analyzer  and  is  held  by  the  observer^ver 
the  eye-piece.  Care  must  be  taken  to  have  the  plane  of  vibration  of  this  nicol  at  right 
angles  to  that  of  the  polarizer,  and  to  leave  sufficient  room  for  the  introduction,  by 
hand,  of  the  test-plate  between  the  eye-piece  and  the  nicol.  With  care  the  plates  can 
be  introduced  with  sufficient  accuracy  to  make  the  test  practical. 


TESTING  FOR    MBRATIOX  DIRECTIONS.      '  3  3 

be  noticed  which  is  either  higher  or  lower  in  the  color  scale  *  than 
the  original  interference  color  of  the  mineral  section.  When  the 
known  directions  of  vibration  of  the  test-plate  are  superposed  over 
corresponding  directions  in  the  mineral  section,  the  effect  is  to 
thicken  the  section  and  the  interference  color  rises  in  the  scale. 

When  the  directions  of  vibration  of  the  test-plate  are  superposed 
over  directions  of  vibration  in  the  mineral  section  which  are  not 
corresponding,  the  effect  is  to  thin  the  section  and  the  interference 
color  sinks  in  the  scale.  Great  care  should  be  taken  when  the 
test-plate  gives  a  higher  interference  color  than  the  mineral  sec- 
tion to  be  tested.  When  this  is  the  case  the  effect  of  the  mineral 
section  on  the  inteiference  color  of  the  test-plate  must  be 
considered. 

For  minerals  which  have  very  strong  double  refraction,  as  zir- 
cons, so  that  the  interference  colors  are  of  the  higher  orders,  it  is, 
advisable  to  use  the  method  of  testing  with  a  quartz  wedge. "f 

If  a  wedge  is  inserted  between  crossed  nicols  with  its  c  direction 
inclined  at  45°  to  the  planes  of  vibration  of  the  nicols,  then  suc- 
cessive interference  colors  will  be  seen  commencing  with  the  gray 
of  the  first  order  and  passing  through  the  colors  as  shown  by  a 
color  chart.  When  moved  in  the  opposite  direction  the  succes- 
sion of  colors  is  reversed.  If  now  the  crystal  section  lies  with  its 
vibration  directions  also  in  the  diagonal  position,  the  color  of  any 
portion  of  the  quartz  wedge  will  be  changed  w'here  it  covers  the 
section,  the  new  color  being  that  of  a  thicker  part  of  the  wedge  if 
the  c  direction  in  the  section  lies  under  the  c  direction  of  the 
wedge.  Also  where  the  wedge  overlaps  the  section  the  displace- 
ment of  the  color  fringes  will  be  towards  the  thin  edge  of  the 
wedge.  On  the  other  hand  the  new  color  will  be  that  of  a  thinner 
part  of  the  wedge  when  the  a  direction  in  the  section  lies  under 
the  c  direction  of  the  wedge.  In  this  case  the  displacement  of  the 
color  fringes  will  be  towards  the  thick  edge  of  the  wedge. 

If  the  wedge  when  inserted  finally  "  compensates  "  the  color  of 
the  crystal  section  (/.  r.,  practically  produces  an  absence  of  color), 

*  A  scale  or  chart  of  interference  colors,  or  the  interference  color  diagram,  should  be 
berore  the  observer  in  order  to  avoid  any  mistakes  as  to  whether  the  new  color  is  higher 
or  lower  in  the  scale. 

f  The  quartz  wedge  is  cut  so  that  one  of  its  faces  is  exactly  parallel  to  the  c  axis 
(hence  also  parallel  to  the  c  vibration  direction)  while  the  other  face  makes  a  very  small 
angle  with  it.     The  direction  c  is  marked  on  the  wedge. 

3 


34       /WESTIGATIOX  OF  CHARACTERS  OF  MIXER ALS. 

then  the  a  in  this  section  must  lie  under  the  c  in  the  wedge,  as  the 
effect  of  the  wedge  has  been  to  continually  thin  the  section. 

Determination  of  Order  of  Interference  Color. 
This  can  be  determined  by  use  of  a  quartz  wedge  or  a  v.  Federow 
mica  wedge.*  Have  the  \-ibration  directions  of  the  given  section 
in  the  diagonal  position,  then  gradually  insert  the  wedge  between 
the  crossed  nicols  so  that  the  corresponding  vibration  directions  in 
the  section  and  wedge  are  crossed,  that  is  so  that  the  colors  are 
run  down  until  finally  dark  gray  or  black  is  obtained.  Count  the 
number  of  times  the  original  interference  color  reappears,  if  n 
times,  then  the  color  is  a  red,   blue,  green,  etc.,  of  ;/  +  i  order. f 

Method  of  Measuring  the  Strength  of  the  Double  Refrac- 
tion BY  von  Federow  Mica  Wedge. | 

"  The  wedge  §  consists  of  fifteen  superposed  quarter  undulation 
mica  plates,  each  about  two  mm.  shorter  than  the  one  beneath  it 
and  with  their  directions  of  vibration  parallel.  The  series  is 
mounted  on  a  strip  of  glass  and  covered  with  a  co\-er  glass. 

The  wave-length  of  a  middle  color  may  be  taken  as  560  fui 
(millionths  of  a  millimeter) ;  hence  each  quarter  undulation  mica 
plate  may  be  considered  to  possess  a  phase  difference  or  retarda- 
tion of  140  ii.fi.  If,  then,  between  the  polarizer  and  analyzer  we 
insert  the  mica  wedge,  so  that  its  direction  of  vibration  of  the 
slower  ray,  c,  is  at  right  angles  to  that  of  a  mineral  under  exam- 
ination, we  subtract  from  the  phase  difference  of  the  mineral  an 
amount  equal  to  ;/  times  140  afi,  in  which  n  represents  the  num- 
ber of  superposed  mica  plates  in  the  field.  When  the  mineral 
appears  dark  the  value  evidently  corresponds  closely  to  the  phase 
difference  of  the  mineral.  || 

From  the  expression  J  =  cX,  in  Avhich  J  is  the  phase  differ- 
ence or  retardation,  X  the  double  refraction,  and  e  the  thickness  in 

*  Described  under  next  test. 

t  In  applying  this  rule  count  the  i°  order  white  as  green  and  the  l°  order  gray  as 
blue. 

+  A.  J.  Moses,    Trans,  X.    Y.,  Acad.  Sci.,  Vol.  XVL,  p.  55,  Jan.,  1S97. 

g  E.  von  Federow,  Zeit.  f.  A'?yst,  etc.,  Vol.  XXXV.,  p.  340,  1S95. 

II  After  the  first  rough  determination  of  the  phase  difference  by  the  mica  wedge,  the 
more  exact  phase  difference  can  be  obtained  by  the  aid  of  a  good  color  chart  or  diagram, 
see  end  of  book. 


METHOD   OF  DETERMIXIXG  MIXERALS.  35 

milHonths  of  millimeters,  the  thickness  can  be  deduced  when  the 
double  refraction  is  known  or  vice  versa.  If  a  mineral  of  known 
double  refraction  can  be  found  in  the  section  near  the  mineral 
under  investigation,*  the  double  refraction  of  the  latter  can  be 
deduced  by  measuring  the  phase  difference  of  the  known  min- 
eral, whence  e  results  and  this  substituted  in  the  above  formula 
yields  AV 

Method  of    Determining    Minerals  and    Thickness  of   Sec- 
tion, BY  Use  of  Table  of  Double  Refraction 
(Maximum)  and  Diagram. f 

Select  some  easily  recognized  mineral  in  the  rock-section  and 
note  the  maximum  interference  color  given  by  any  of  its  sections. :|: 
The  strength  of  the  double  refraction  of  this  mineral  being  known, 
look  up  on  the  diagram  the  diagonal  line  corresponding  to  this 
double  refraction  and  follow  along  this  line  toward  the  left  hand 
lower  corner  until  the  observed  interference  color  is  reached,  when 
the  horizontal  line  will  indicate  the  thickness  of  the  section.  The 
thickness  is  given  in  hundredths  of  millimeters.  Then  in  the  case 
of  the  unknown  mineral  pick  out  the  section  giving  the  highest 
interference  color  and  carry  along  the  same  horizontal  line  until 
this  new  color  is  located,  §  then  pass  up  to  the  right  along  the 
diagonal  line  to  the  numbers  indicating  the  strength  of  the  double 
refraction  of  the  unknown  mineral.  Turn  to  the  table  where  the 
minerals  will  be  found  having  about  this  strength  of  double  re- 
fraction. 

Example :  In  a  section  of  granitite  (biotite-granite)  a  grain  of 
quartz  was  selected,  which  gave  the  brightest  color.  This  color 
was  a  bright  grayish-white  and  the  known  double  refraction  of 
quartz  is  0.009.  Following  down  on  this  diagonal  line  until  the 
interference  color  was  reached  it  was  seen  that  the  thickness  of  the 
section  was  about  0.015  "^"''-  (^  ^'^''y  thin  section).  The  section 
of  the  undetermined  mineral,  giving  the  highest  order  color,  showed 

*  It  is  not  safe  to  use  minerals  near  the  edge  of  the  section,  as  the  thicknesses  are 
apt  to  be  unequal. 

•)■  See  at  end  of  appendix. 

X  In  this  way  eliminate,  so  far  as  possible,  the  effect  of  the  orientation  of  the  mineral 
section. 

I  The  different  mineral  sections  are  all  supposed  to  have  the  same  thickness  through- 
out the  rock-section. 


6      IXVESTIGATIOX  OF  CHARACTERS  OF  MINERALS. 


a  bright  purple-blue.  Passing  along  the  0.015  horizontal  line 
until  this  color  was  reached  and  then  up  along  the  diagonal  line, 
it  was  seen  that  the  double  refraction  of  this  mineral  must  be  about 
0.041.      The  table  gives  musco\ite  and  aegerite  as  having  about 


Fig.  18.  —  Sanidine  crystal  f,  showing  Carlsbad  twin  (which,  as  it  consists  of  two 
parts  only,  may  be  called  simple),  and  quartz  cj  in  rhyolite. 

this  double  refraction.  The  mineral  was  proved  to  be  muscovite 
by  its  absence  of  color  and  relief  and  by  the  characteristic  cleavage 
and  parallel  extinction. 

Structure : 

{a)  Ti^'iiining,  generally  noticed  by  the  parts  of  the  twin  not  ex- 
tinguishing at  the  same  time.  It  may  also  be  observed,  without 
crossed  nicols,  just  as  in  the  case  of  macroscopic  minerals. 


Fig.    19.  —  Microcline,  showing  crossed  or  "gridiron"  twinning.      Crossed  nicols. 


STRUCTURE.  37 

Twinning  may  be  described  as  :  simple,  Fig.  1 8  ;  poly  synthetic, 
due  to  repeated  twinning  after  the  same  law,  Fig.  1 2  ;  and  crossed 
or  '^gridiron,''  due  to  repeated  twinning  after  two  laws,  Fig.  19. 

ili)  Zonal  structure,  often  only  made  visible  by  the  zones  extin- 


FiG.  20.  —  Zonal  feldspar  (Carlsbad  twin)  in  trachyte.     Crossed  nicols. 

guishing  at  different  times.  It  may,  however,  be  noticed  by  the 
zones  being  of  slightly  different  color,  or  by  the  zonal  distribution 
of  inclusions.  In  the  case  of  the  feldspars  the  zonal  structure  may 
be  caused  either  by  the  cr}'stal  being  formed  of  zones  of  different 


Fig.     21.  —  Sphrerulites    in     felsite.      Ground-mass     shows     aggregate    structure. 
Crossed  nicols. 

chemical  composition  (the  successive  zones  in  the  plagioclases 
growing  more  acid  towards  the  exterior),  or  by  ultra-microscopic 
twinning,*  Fig.  20. 

'''Yidi.rVtx^?,  Petrology  for  Students,  1895,  p.  14. 


38       IXVESTIGATION  OF  CHARACTERS  OF  MINERALS. 

(r)  Aggregate  structure,  being  a  confused  mass  of  separate  little 
crystals,  scales  or  grains  all  extinguishing  at  different  times,  Fig.  2 1 . 

{d)  Sphcerulitic  structure,  produced  by  the  aggregation,  in  a 
radiate  form,  of  crystals  or  ciystallites.  It  is  generally  easily  per- 
ceiv^ed  by  the  dark  cross,  resulting  from  the  extinguishing  of  the 
light  in  those  crystals  whose  directions  of  vibration  are  parallel  to 
the  planes  of  vibration  of  the  nicols.  When  the  stage  is  revolved 
the  arms  of  the  cross  do  not  rotate,  Fig.  2 1 . 

{c)  PscudojHorphic  structure,  which  may  be  partial  or  complete 
and  is  noticed  by  the  changed  portions  producing  different  optical 


Fig.  22.  —  Olivine  decomposed  to  serpentine.  The  pseudomorphism  has  been  quite 
complete,  only  small  portions  of  the  original  olivine  remaining.  The  outline  of  the 
parent  crystal  can  be  quite  distinctly  seen.     Crossed  nicols. 

effects  from  those  of  the  original  mineral.  Sometimes,  although 
the  pseudomorphism  has  been  quite  complete,  the  form  of  the 
original  mineral  or  crystal  may  still  be  seen.  Fig.  22. 

Characters  Observed  by  Convergent  Light. 
Convergent  light  is  obtained  by  passing  the  ra)\s  of  polarized 
light  through  a  strong  condensing  lens,  which  generally  fits  like  a 
cap  over  the  top  of  the  polarizer.  By  means  of  a  suitable  adjust- 
ment the  condensing  lens  can  be  brought  very  close  to  the  lower 
surface  of  the  section  on  the  stage.  The  lens  thus  sends  a  cone 
of  light  through  the  section,  and  used  in  connection  with  erossed 
nicols  a  series  of  optical  phenomena,  called  interference  figures,^ 
are  produced. 

*Iddings'  Rosenbusch,  p.  67.      Moses'  Characters  of  Crystals,  p.   1 15. 


UXIAXIAL  IXTERFEREXCE  FIGL'RES.  39 

Each  direction  in  which  rays  are  sent  is  traversed  by  a  minute 
bundle  of  parallel  rays  and  these  rays  extinguish  and  produce  in- 
terference colors  as  already  described  for  parallel  light.  Hence 
each  direction  yields  a  spot  or  picture  in  the  field  of  view  and  from 
all  these  spots  combined  there  results  an  "  interference  figure  "  or 
picture,  depending  upon  the  structure  of  the  section  for  all  the 
directions  traversed  by  the  rays. 

A  very  high  power  objective  *  must  be  used,  and  when  the  eye- 
piece is  removed,  a  small  image  of  the  interference  figure  will  be 
seen.  In  some  microscopes  an  arrangement  is  made  for  getting  a 
magnified  image  of  the  interference  figure,  by  retaining  the  eye- 
piece and  using  an  additional  Bertrand  lens. 

In  order  to  get  good  results  care  must  be  taken  to  have  strong 
illumination  and  the  condensing  lens  close  up  under  the  section. 
The  tests  are  best  made  with  monochromatic  light,  but  with  .white 
light  the  effects  are  substantially  the  same,  the  only  difference 
being  that  the  rings  and  curves  are  variously  colored  instead  of 
being  simply  light  and  dark. 

Isotropic  substances  show  no  iiitcrft^rcncc  figures. 

Uniaxial  Interference  Figures. 

{a)  Sections  perpendicular  to  the  optic  or  vertical  axis  c  show  a 
dark  cross,  with  or  without  colored  rings,  Figs.  23  and  24.      The 


Fig.  23.  Fig.  24. 

figure  is  symmetrical  to  the  centre,  as  the  optical  behavior  of  uni- 
axial cr)^stals  is  symmetrical  to  the  optic  axis. 

The  arms  of  the  cross  are  parallel  to  the  planes  of  vibration  of 
the  nicols,  and  the  figure  does  not  move  when  the  stage  canying 
the  section  is  rotated,  f 

*  In  the  Seibert  microscope  use  No.  V.  objective,  in  Fuess  microscope  No.  7  objec- 
tive, and  in  English  microscopes  a  \"  or  \"  objective. 

t  Each  convergent  ray  will  have  its  vibration  direction  either  in  or  at  90°  to  the  plane 
through  the  ray  and  the  optic  axis.  Hence  all  rays  vibrating  parallel  to  the  vibration 
planes  of  both  nicols  will  be  completely  cut  out.  As  the  section  is  rotated  new  rays  suc- 
cessively come  into  these  positions,  so  the  same  effect  is  maintained. 


40       IXVESTIGATIOX  OF  CHARACTERS   OF  MINERALS. 

[p)  Sections  oblique  to  the  optic  axis  show  a  portion  of  a  dark- 
cross,  with  or  without  colored  rings,  Fig.  25. 

The  centre  of  the  cross  is  not  in  the  axis  of  rotation,  and  as  the 
stage  bearing  the  section  is  revolved,  the  centre  of  the  cross  de- 
scribes a  circle,  the  arms  always  maintaining  parallel  positions. 

If  the  section  is  still  more  oblique  to  the  optic  axis  the  centre 
of  the  interference  cross  may  be  outside  the  field  of  view,  and  only 


Fig.  25. 

the  dark  arms  will  be  seen  swinging  past,  when  the  section  is  ro- 
tated, thus  making  the  figure  rather  indefinite. 

Sections  parallel  to  an  optic  axis  show  hyperbolic  curves,  which 
might  be  confused  with  a  biaxial  interference  figure  with  axial  angle 
of  180°. 

Sections  which  are  thick  and  have  strong  double  refraction  will 
show  the  cross  and  rings  clearly  and  sharply  defined,  there  being 
quite  a  number  of  rings  crowded  close  together.  Sections  which 
are  very  thin  and  have  weak  double  refraction  show  only  a  broad 
dark  cross  and  no  rings.  The  interference  figures  will  vary  be- 
tween these  extremes,  depending  on  the  thickness  of  the  section 
and  the  strength  of  the  double   refraction. 

To  obtain  the  most  characteristic  figures,  observations  must  be 
made  on  sections  about  perpendicular  to  the  optic  axis,  that  is 
sections  which  remain  dark  or  nearly  dark  during  complete  rotation 
between  crossed  nicols  in  parallel  light. 

Optical  Character,  Positive  or  Negative.  After  having  ob- 
tained a  uniaxial  interference  figure,  test  it  by  means  of  a  ]^  undu- 
lation mica  plate.  This  plate  must  be  introduced  between  the  ob- 
jective *  and  the  anah'zer  in  such  a  way  that  its  vibration  direction  r, 
marked  on  the  plate,  makes  an  angle  of  45°  with  the  planes  of 
vibration  of  the  nicols. 

*  In  the  Seibert  microscope  there  is  a  little  slot  /-  for  this  purpose  just  above  the 
objective. 


OPTICAL   CHA RA  CTER. 


41 


When  this  is  done  the  inteiference  figure  changes,  or  may  more 
or  less  disappear,  two  dark  spots  or  blotches  being  brought  prom- 
inently into  view.  If  rings  are  still  seen  it  will  be  noticed  that 
they  have  expanded  in  the  quadrants  occupied  by  the  dark  spots, 
and  have  contracted  in  the  remaining  quadrants.  This  fact  may 
make  it  possible  to  determine  the  optical  character  of  a  section, 
which  is  so  oblique  to  the  optic  axis  that  the  dark  spots  are  not 
seen  after  the  introduction  of  the  mica  plate. 

If  the  optical  character  is /^j-zV/tr  the  line  joining  these  dark  spots 
is  perpendicular  to  the  direction  C  of  the  mica  plate,  see  Fig.  26. 


Fig.  26. 

If  ncgcxtive  the  line  joining  the  dark  .spots  coincides  with  the  di- 
rection  c  of  the  mica  plate,  see  Fig.  27. 

The  (-(-)  and  ( — )  character  is  easily  determined  by  remember- 
ing that  the  line,  joining  the  dark  spots,  makes  the  +  and  —  sign 
respectively  with  the  direction  C  of  the  mica  plate.  The  direction  C 
■of  the  mica  plate  (represented  in  the  figures  by  an  arrow)  is  of  course 
not  seen,  but  its  position  must  be  borne  in  mind  when  making  this 
test.  This  test  can  be  made  with  either  monochromatic  or  white 
light. 

If  the  mica  plate  does  not  give  satisfactory  results,  which  will  be 
the  case  when  the  broad  cross  of  a  weak  doubly  refracting  crystal 
is  to  be  tested,  use  a  selenite  plate,  cut  the  proper  thickness  to 
give  the  red  color  of  the  first  order. 

This  plate  must  be  introduced  with  its  vibration  direction  a 
(previously  determined)  making  an  angle  of  45°  with  the  planes 
of  vibration  of  the  nicols.  Instead  of  the  dark  spots  being  seen 
there  will  appear  two  blue  and  two  red  quadrants.  The  diagonally 
opposite  quadrants  being  of  the  same  color. 

In   determining   the  (-|-)  and   ( — )  character  consider  the  blue 


42       IXVESTIGATION  OF  CHARACTERS  OF  MINERALS. 

quadrants  as  the  equivalent  of  the  dark   spots  in  the  preceding 
case.      This  test  must  be  made  with  white  hght.* 

Biaxial  Interference  Figures. 

(c?)  Sections  perpendicular  to  an  optic  axis  exhibit  the  interfer- 
ence figures  shown  in  Figs.  28  and  29,  the  curves  being  nearly 
circular  and  a  straight  black  bar  bisecting  these  curves,  whenever 
the  trace  of  the  plane  of  the  optic  axes  coincides  with  the  vibration 


Fig.  28. 


Fig.  29. 


direction  of  either  nicol.  As  the  stage,  carrying  the  section,  is  ro- 
tated the  bar  changes  into  one  arm  of  a  hyperbola  and  back  again 
into  a  bar.  This  arm  or  bar  will  rotate  in  the  opposite  direction 
to  the  stage. 

As  previously  stated  sections  of  biaxial  crystals,  perpendicular 
to  an  optic  axis,  do  not  remain  dark  during  rotation  of  the  stage 
between  crossed  nicols  in  parallel  light.  On  the  contrary  these 
sections  remain  uniformly  illuminated  or  show  a  color  tint.f 


Fig.  30. 


Fig.  31. 


{U)  Sections  perpendicular  to  the  acute  bisectrix  (see  p.  5),  ex- 
hibit interference  figures  like  those  shown  in  Figs.  30  and  31. 
Fig.  30  shows  the  appearance  of  the  interference  figure  when 

*The  optical  character  may  also  be  determined  in  parallel  light  by  proving  c  ^=  t 
(-)-),  c  z=(y  ( — ).  The  optical  character  of  the  principal  zone  or  the  sign  of  the  elonga- 
tion is  often  given  in  tables.  This  optical  character  or  sign  is  ( +  )  when  the  principal 
zone  axis  or  the  direction  of  elongation  is  parallel  to  c  and  ( — )  when  parallel  to  a. 

f  Moses'  Characters  of  Crystals,  p.  136. 


BIAXIAL  INTERFERENCE  FIGURES.  43 

the  plane  of  the  optic  axes  is  parallel  to  the  plane  of  vibration  of 
either  nicol,  and  Fig.  3 1  shows  the  appearance  when  this  plane  is 
inclined  45 '^  to  the  planes  of  vibration  of  the  nicols. 

As  the  stage,  carrying  the  section,  is  rotated  the  dark  cross 
seems  to  dissolve  into  the  two  branches  of  a  hyperbola,  which 
again  unite  to  form  a  cross. 

In  sections  perpendicular  to  a  bisectrix,  with  a  large  axial  angle, 
the  figure  will  appear,  during  a  rotation  of  90°  (in  the  direction  of 
the  hands  of  a  watch),  as  in  Fig.  32,  top  row.  When  the  section 
is  somewhat  oblique  to  an  "optic  axis,"  the  figure  appears  as  in 
middle  row  ;  and  when  still  more  oblique,  as  in  bottom  row. 


Fig.  32.  —  Biaxial  Interference  Figures  (  from  Reinisch).  Top  row  :  Almost  perpen- 
dicular to  bisectrix,  large  axial  angle.  Middle  row  :  Somewhat  oblique  to  an  "optic 
axis."      Bottom  row  :   More  oblique  to  an  "optic  axis." 

The  black  centres  *  of  the  small  ellipses  and  the  black  hyperbolic 
curves  mark  the  points  of  emergence  of  the  optic  axes,  and  therefore 
indicate  approximately  the  size  of  the  axial  angle,  2E. 

Sections  in  other  positions,  relative  to  the  optic  axes,  give  inter- 
ference figures  less  definite  in  appearance  than  those  just  described  ; 
and  the  same  conditions  affect  the  appearance  of  all  figures  as  in 

*  This  assumes  the  optic  axes  for  different  colors  to  emerge  about  at  the  same  points. 
If  there  is  marked  "dispersion  "  the  black  bands  and  hyperbolas  may  be  rainbow-hued, 
as  with  titanite. 


44       LW'ESTIGATIOX  OF  CHARACTERS  OF  MIXERALS. 

the  case  of  uniaxial  crystals.  Very  thin  sections,  of  weak  double 
refraction,  may  only  show  indistinct  dark  crosses  or  hyperbolic 
cur\'es.  without  an\'  ellipses. 

The  section  perpendicular  to  the  acute  bisectrix,  which  gives  the 
most  characteristic  interference  figure,  cannot  generally  be  recog- 
nized except  by  an  examination  in  convergent  light.  Therefore  no 
clue  can  be  obtained  as  to  the  best  section  to  test,  and  the  safest 
method  is  to  test  all  the  sections  of  the  mineral  occurring  in  the 
rock  section. 

It  must  be  remembered  that  this  uncertainty,  in  the  choice  of 
sections  for  testing,  does  not  exist  in  uniaxial  crystals  ;  where  the 
best  sections  are  indicated  by  the  fact  that  they  remain  dark  or 
nearly  so  during  complete  rotation  between  crossed  nicols. 

The  uniaxial  or  biaxial  character  of  a  mineral  section,  which  only 
shows  an  indistinct  bar,  may  be  determined  as  follows  :  A  bar  (one 
arm  of  the  cross)  of  a  uniaxial  interference  figure  moves  in  the 
same  direction  as  the  rotating  stage,  and  ahva\-s  remains  straight, 
while  the  biaxial  bar  rotates  in  the  opposite  direction  to  the  stage 
and  becomes  cur\'ed. 

Optical  Character,  Positive  or  Negative.  When  the  axial 
angle  is  very  small,  so  that  the  interference  figure  approaches  that 
of  a  uniaxial  crystal^  the  methods  used  for  testing  uniaxial  figures 
are  employed. 

When,  however,  the  axial  angle  is  large,  the  following  method 
can  be  used  : 

After  having  obtained  an  interference  figure,  from  a  section  as 
nearly  at  right  angles  to  the  acute  bisectrix  *  as  possible,  the  stage 
is  rotated  until  the  plane  of  the  optic  axes  (the  trace  of  which  on 
the  plane  of  the  section  is  the  line  joining  the  points  of  emergence 
of  the  two  optic  axes)  makes  an  angle  of  45°  with  the  planes 
of  vibration  of  the  crossed  nicols  or  the  cross-wires  in  the  eye- 
piece. 

A  quartz  wedge  f  is  now  pushed  in  between  the  mineral  section 

*  The  interference  figure,  perpendicular  to  the  obtuse  bisectric,  would  be  of  the  same 
type  with  a  larger  axial  angle.  Ordinarily  this  figure  would  not  come  within  the  limits 
of  the  field  of  view  of  the  microscope.  Confusion  may  arise,  however,  but  in  a  section 
perpendicular  to  the  acute  bisectric  the  cross  dissolves  more  slowly  into  the  hyper- 
bolas than  in  the  case  of  a  section  perpendicular  to  the  obtuse  bisectric.  At  times  it 
may  be  necessary  to  measure  the  axial  angle  to  be  sure. 

f  For  construction  of  quartz  wedge,  see  p.  t^t,. 


DETERMIXATIOX  OF  THE  AXIAL  AXGLE.  45 

and  the  analyzer,  *  so  that  its  axis  r  =  c  (previously  determined 
and  marked  on  the  wedge)  is  either  at  right  angles  or  parallel  to 
the  plane  of  the  optic  axes  of  the  mineral  section. 

The  optical  character  of  the  mineral  is  positive  when  the  ellipses, 
surrounding  the  points  of  emergence  of  the  two  optic  axes,  appear 
to  expand  or  open  out  when  the  quartz  wedge  is  pushed  in  with  its 
axis  parallel  to  the  plane  of  the  optic  axes. 

The  optical  character  is  negative  when  the  ellipses  appear  to 
expand  or  open  out  when  the  wedge  is  pushed  in  with  its  axis  at 
right  angles  to  the  plane  of  the  optic  axes. 

As  the  ellipses  expand  they  move  from  the  points  of  emergence 
of  the  optic  axes  towards  the  center  of  the  interference  figure,  and 
finally  open  into  lemniscates  which  move  outward  from  the  plane 
of  the  optic  axes. 

Even  when  the  section  is  very  thin  and  the  double  refraction 
very  weak,  only  the  black  hyperbolas  without  ellipses  being  seen, 
the  test  can  be  made ;  and  colored  ellipses  will  appear,  after  the 
pushing  in  of  the  quartz  wedge,  which  will  act  in  the  same  way  as 
the  ellipses  of  the  interference  figure. 

In  a  section  at  right  angles  to  the  obtuse  bisectric  these  results 
are  all  reversed. 

Determination  of  the  Axial  Angle,  t  This  can  be  approxi- 
matel}-  determined  with  a  petrographical  microscope,  if  equipped 
with  a  micrometer  eye-piece.  Have  the  axial  plane  of  the  crystal 
section  in  the  diagonal  position,  Fig.  3 1  ;  and  measure  the  distance 
d  from  the  centre  to  either  hyperbola  with  a  micrometer  (or  average 
the  distance  to  both).  Then  sin  E^djC,  in  which  C  is  a  con- 
stant for  the  same  combination  of  lenses  and  is  obtained  by  using 
a  cr}'stal  section  (mica  cleavage)  of  known  axial  angle.  For 
example,  in  a  mica  with  2E  ^=  91°  50'  and  d  =^  41 -5  divisions  on 
the  micrometer  scale,  C  =■  dj sin  £=  57.78  for  that  special  combi- 

*The  wedge  can  be  introduced  in  either  of  the  several  ways  described  for  the  intro- 
duction of  the  test  jilates  on  p.  32. 

f  For  other  methods  of  measuring  the  axial  angle,  see  A.  J.  Moses,  Characters  of 
Crystals,  Chap.  XL,  pp.   148-153. 

For  convenience  in  many  cases  only  zE  is  recorded,  as  then  an  indication  is  given 
as  to  whether  the  axial  angle  is  visible  with  an  ordinary  microscope  (arranged  for 
observation  with  convergent  light  for  interference  figures).  If  2.E  is  very  large  the 
axial  angle  can  only  be  observed  by  covering  the  section  with  some  transparent,  strongly 
refracting  fluid.  For  the  Seibert  microscope  with  objective  V  the  limit  for  good 
results  is  about  2^=  90°-ioo°. 


46       IXrESTIGATIOX  OF  CHARACTERS  OF  MINERALS. 

nation  of  lenses.      The  true  axial  angle  can  be  obtained  from  the 
equation  2  /  '=  d'^C. 

Optical  Distinctions  between  Orthorhombic,  Monoclinic,  and 
Triclinic  Crystal  Sections  (perpendicular  to  acute  and  obtuse 
bisectrices).  The  interference  figures  are  always  symmetrical 
in  shape  and  distribution  of  color  to  the  planes  and  axes  of  sym- 
metry of  the  crystal  system  ;  hence  are  most  symmetrical  in  the 
othorhombic,  less  so  in  the  monoclinic  and  still  less  so  in  the  tri- 
clinic s)'stem. 

OrtliorJiombic  crystals  show  the  figures  always  in  two  of  the 
pinacoids  and  in  white  light  the  color  distribution  will  be  symmet- 
rical to  the  trace  of  the  axial  plane  and  the  line  through  the  centre 
at  right  angles  to  this  trace  and  also  to  the  central  point. 

Monoclinic  crystals  show  the  figures  in  the  clino  pinacoid  or  in 
sections  at  right  angles  to  this.  In  white  light  the  color  distribu- 
tion is  never  symmetrical  to  two  lines,  but  is  symmetrical  either 
to  the  trace  of  the  axial  plane  {inclined  dispersion  *),  or  to  the  line 
through  the  centre  at  right  angles  to  this  trace  [Jwrizontal  disper- 
sion), or  to  the  central  point  {crossed  dispersioti). 

Triclinic  crystals  show  in  white  light  figures  with,  distribution  of 
color  unsymmetrical  to  any  line  or  point. 

In  white  light  the  "  color  fringes  "  are  due  to  the  "  dispersion  "  * 
of  the  optic  axes  and  bisectrices.  That  is  for  each  color  (for  light 
of  each  wave-length)  there  is  a  particular  interference  figure  ;  the 
overlapping  of  these  superposed  figures  producing  the  color 
fringes. 

When  the  axial  angle  is  larger  for  red  light  than  for  violet,  the 
dispersion  is  said  to  be  <>  >  0  and  the  interference  figure,  in  the 
position  of  Fig.  31,  will  show  the  hyperbolic  curves  fringed  with 
red  towards  the  centre  (inside).  In  general  the  color  with  the 
larger  axial  angle  is  nearer  the  centre  of  the  field.  This  is  due  to  the 
extinguishing  of  light  of  each  color  at  the  axial  points,  the  resulting 
colors  at  these  points  being  produced  by  white  light  minus  the 
absorbed  color.  When  the  disper.sion  is  ^  >  y  the  rev^erse  distri- 
bution of  color  fringes  will  take  place. 

By  measuring  the  axial  angle  in  red  and  blue  light,  this  disper- 
sion of  the  optic  axes  can  also  be  obtained. 

*For  dispersion,  etc.,  see  A.  J.  Moses'  Characters  of  Crystals,  p.  140. 


RESUME  OF  USES  OF  LIGHT.  47 

Resume  of  the  Uses  of  Parallel  and  Convergent  Light. 

Parallel  light  is  used  to  distinguish  between  isotropic  and  aniso- 
tropic substances,  to  locate  directions  of  vibrations,  to  measure 
extinction  angles  and  to  find  the  directions  of  vibration  of  the 
faster  and  slower  rays. 

Convergent  light  is  used  to  distinguish  between  uniaxial  and  bi- 
axial crystals,  to  determine  whether  a  section  that  appears  to  be 
isotropic  is  really  so  or  only  perpendicular  to  an  optic  axis  and  to 
determine  the  optical  character,  grade  of  symmetry  (system),  axial 
angle  and  dispersion. 


CHAPTER    IV. 
The  Microscopic  and  Optical  Characters  of  Minerals. 

OPAL. 

Isotropic.  Amorphous. 

Composition  :  SiOj./^H^O,  generally  soluble  in  caustic  alkalies. 

Usual  Appearance  in  Sections  :  Colorless  patches  or  veins,  also  at  times  with 
sphserulitic  structure,  showinf;  interference  cross  between  crossed  nicols.  Often  shows 
anomalous  double  refraction  due  to  strains.  The  refractive  index  is  very  low  (1.46)  so 
that  the  surface  of  the  opal  appears  rough. 

Remarks  :  Found  as  a  secondary  mineral  in  many  acid  volcanic  rocks,  rhyolite, 
trachyte,  andesite,  etc.,  and  also  in  basic  basalts.      H.,  5.5  to  6.5.      Sp.  gr.,  2.2. 

LIMONITE. 

Amorphous. 

CoMPOSiTiO.x  :   Fe.,(OH)g,  Fe.^Og,  frequently  quite  impure. 

Usual  Appearance  in  Sections  :  Brownish  and  opaque,  in  very  thin  sections  may 
be  translucent. 

Remarks  :  Limonite  is  essentially  a  decomposition  product,  often  forming  pseudo- 
morphs  after  ferruginous  silicates  or  halos  about  the  iron  ores. 

PYRITE,  Pyrites. 

ISO.METRIC. 

Composition  :  FeS.,. 

Usual  Appearance  in  Sections :  Cubes,  pentagonal  dodecahe- 
drons, combinations  of  these  forms  ;  or  in  irregular  grains.  Out- 
line of  cross-sections  generally  square. 

Opaque,  and  by  reflected  light,  bright  yellow,  with  strong 
metallic  lustre. 

Alters  very  easily  to  the  oxides  of  iron  (rust). 

Remarks  :  May  be  present  in  all  kinds  of  rocks,  and  abundant  in  igneous  and  sedi- 
mentary rocks.  Not  noticeably  acted  on  by  hydrochloric  acid.  H.,  6  to  6.5.  Sp.  gr., 
4.9  to  5.2. 

PYRRHOTITE,  Magnetic  Pyrites. 

Composition  :  Fe^S,  to  Fe^S,.,.  Distinguished  from  pyrite  by  being  practically 
always  in  irregular  masses  and  not  in  crystals,  and  by  bronze  yellow  color  with  reflected 
light.      Found  in  basic  eruptive  rocks,  more  rarely  in  schists. 

MAGNETITE,  Magnetic  Iron  Ore. 

Isometric 
Composition  :   Fe.^O^,  often  contains  Ti. 

4  49 


50 


CHARACTERS  OF  MINERALS. 


Usual  Appearance  in  Sections  :  Grains  and  crystals  (generally 
octahedra),  Fig.  i},  \\.  Skeleton  crystals  frequent  in  highly  fer- 
ruginous eruptive  rocks. 

Twimiiiig. — Common,  according  to  Spinel  law. 

Opaque,  and  by  reflected  light,  bluish-black,  with  strong  metallic 
lustre. 

Distinguished    from :      Hematite,    Chromite,     Ilmenite    and 


Fig.  33.  — A,  Zircon  crj'stals  (isolated  from  granite)  in  balsam,  showing  high  relief. 
B,  Magnetite  crystals.  C,  Ilmenite,  showing  partial  decomposition  to  leiicoxene  along 
crystallographic  directions. 

Graphite,  by  being  easily  separated  from  powdered  rock  by  weak 
magnet. 

Remarks  :  Very  widely  distributed  in  eruptive  rocks  and  crystalline  schists.  In 
the  eruptive  rocks  magnetite  belongs  to  the  oldest  secretions  from  the  magma,  immedi- 
ately followed  by  chrysolite,  biotite,  hornblende,  augite,  etc. ;  hence  often  appears  as 
inclusions  in  these  and  other  minerals.  Magnetite  grains  may  form  with  other  sub- 
stances pseudomorphs  after  hornblende,  biotite,  hypersthene,  etc.  Such  pseudomorphs 
appear  to  be  caused  by  "resorption."  Magnetite  is  strongly  magnetic  and  soluble  in 
hydrochloric  acid.      H.,  5.5  to  6.5.      Sp.  gr.,  4.9  to  5.2. 

CHROMITE. 

Isomp:tric. 
Composition  :  FeCr.^O,. 

Usual  Appearance  in  Sections  :  Octahedral  crystals,  grains  and  in  the  olivine  rocks 
sometimes  in  dense  aggregates.  May  be  surrounded  by  green,  pleochroic  halo  of  chrome 
ochre. 

Opaque,  and  by  reflected  light,  brownish-black  to  black,  with  general  absence  of 
metallic  lustre.  Usually  translucent  and  brownish  on  the  edges  (by  transmitted  light), 
with  a  very  rough  surface  due  to  high  index  of  refraction  («  ^  2.  i ). 

Distinguished  from : 

(«)   M.Aii.M'.ri  IK  by  brownish-black  to  black  color  and  genera)  absence  of  metallic 


GARNET.  51 

lustre  (by  reflected  light)   and  by  grains  being  usually  translucent  and  brownish  on  the 
edges  (by  transmitted  light). 

{b)    Spinel  (Picotite),  see  under  Spinel. 

Remarks  :  Common  in  crystalline  rocks,  rich  in  magnesia,  and  in  serpentine. 
Chromite  is  not  acted  on  by  acids,  is  non-magnetic  and  gives  chromium  bead  test.  H., 
5.5.      Sp.  gr.,  4.3  to  5.6. 

SPINEL. 
Isotropic.  Isometric. 

Composition:  Mg(A102).^.     Pleonaste  (Fe,  Mg  spinel),  Picotite  (Cr  spinel). 

Usual  Appearance  in  Sections  :  Octahedral  crystals  and  twins  (after  spinel  law), 
less  often  in  grains.  Always  optically  normal  and  never  decomposed  in  rocks.  Usually 
colorless  or  dark  green  (pleonaste)  to  brown  (picotite).  The  refractive  index  is  high 
(«  =  1.72,  spinel  proper,  to  2.00,  chrome  spinel),  hence  the  relief  is  marked  and  the 
surface  rough. 

Distinguished  from ; 

(«)  Garnet  when  colorless  by  octahedral  shape  of  crystals  (garnet  forms  being  no 
and  211),  when  brown  (picotite)  from  melanite  garnet  by  common  zonal  coloration 
of  the  latter,  but  may  be  only  possible  by  chemical  reactions.  Furthermore  spinel  may 
have  green  color  and  is  never  decomposed. 

(<J)    Perovskite  by  the  lower  index  of  refraction  and  the  absence  of  reaction  for  Ti. 

(r)   Chromite  chemically  or  by  density  or  hardness. 

Remarks  :  Found  in  gneiss,  granulite,  Iherzolite  and  in  regions  of  contact  meta- 
morphism  and  secondary  bedding  formations  (picotite),  olivine-basalt  and  serpentine. 
Spinels  are  insoluble  in  hydrochloric  acid.      H.,  8.      Sp.  gr.,  3.5  to  4.1. 

GARNET. 

Isotropic.  Isometric. 

Composition  :  ^\  R'"^  (SiO  J3.  R"  is  Ca,  Mg,  Fe  or  Mn  ;  R'" 
is  Al,  Fe'",  or  Cr,  rarely  Ti. 

Usual  Appearance  in  Sections:  Irregular  grains,  Fig.  13,  or 
simple  crystals,  showing  forms  ocO  (i  10)  and  2O2  (211),  alone  or 
in  combination,  Fig.  14  b.  Zonal  structure  not  infrequent,  espe- 
cially in  the  titanium  varieties.  Fig.  34. 


Fig.  34. — Garnet,  with  zonal  strucuiic,  in  gneiss       (From  Cohen.) 


5  2  CHA  RA  C  TKRS  OF  MINE  RA  LS. 

Color.  —  Colorless,  or  nearly  so,  to  yellowish,  reddish  or 
brownish. 

Judex  of  Refraction.  — //  =  1.7 50- 1.8  56,  hence  relief  high  and 
surface  very  rough. 

Fracture.  —  Irregular  cracks  occur,  but  no  cleavage  noticed. 

Crossed  Nicols :  As  garnets  are  isotropic,  sections  remain  dark 
during  complete  rotation.  Optical  anomalies  may  however  occur, 
but  are  generally  confined  to,  titanium  free,  lime  garnets  and  man- 
ganese garnets.  The  effect  being  to  divide  the  crystal  sym- 
metrically into  different  areas,  "  dodecahedral  structure." 

Alteration  :  Garnets  are  usually  fresh,  but  may  be  found  altered 
to  chlorite  or  hornblende. 

Distinguished  From  :  Spinel  and  Perovskite.  —  See  under  the 
latter. 

Remarks  :  Found  principally  in  granulites,  metamorphic  rocks,  contact  rocks, 
crystalline  schists,  etc.  Certain  varieties  may  be  found  in  eruptive  rocks  or  olivine 
rocks.  May  form  pegmatitic  borders  with  pyroxene,  spinel,  etc.  Garnets  are  prac- 
tically insoluble  in  hydrochloric  acid.  H.,  6.5  to  7.5.  Sp.  gr.,  3.4  to  4.3.  The 
insolubility  in  acids  and  the  high  sp.  gr.  help  in  separating  garnet  from  a  powdered  rock. 

LEUCITE. 

Isotropic.  Isometric* 

Composition  :  KAl(Si03)2. 

Usual  Appearance  in  Sections  :  Crystals  or  grains,  which  vary 
greatly  in  size.  Cross-sections  often  nearly  round.  When  very 
small  and  free  from  inclusions  may  be  easily  overlooked.  Some- 
times the  grains  are  surrounded  by  tangentially  arranged  needles 
of  different  minerals. 

Color. —  Colorless. 

Index  of  Refraction. —  ;/  =  1.509,  hence  no  relief  and  generally 
smooth  surface. 

Fracture. —  May  be  noticed,  but  no  cleavage  observed. 

Inclusions. —  Common,  radially  or  zonally  arranged,  consisting 
of  minerals  or  glass.  Fig.  35. 

*  The  system  of  crystallization  of  leucite  has  been  the  subject  of  much  discussion. 
Its  habit  is  isometric.  The  consensus  of  opinion  seems  to  be  that  leucite  crystallizes  in 
the  isometric  system,  but  that  the  isometric  molecular  arrangement,  at  least  of  the 
larger  crystals,  cannot  exist  for  the  temperature  and  pressure  at  the  earth's  surface. 
Hence  molecular  displacement  takes  place,  giving  rise  to  a  more  or  less  complicated 
apparent  twinning,  and  optical  anomalies  are  noticed.  The  isotropic  character  returns 
if  the  section  is  heated  to  500°C.      Iddings''  Roseultiiscti,  p.   133. 


LEI  CITE.  53 

Crossed  Nicols :  The  smaller  crystals  appear  isotropic  ;  the  larger 
crystals  show  characteristic  intersecting  systems  of  twin  lamellae, 
Fig.  36. 

Double   Refraction. — Very    weak    (/' —  «  =  o.ooi).      In    thin 


Fig.  35. — Leucite,  with  radial  and  tangential  inclusions,  Vesuvius  Lava.      (From 
Cohen. ) 

sections  it  may  be  necessary  to  use  a  sensitive  color-plate  to  prove 
double  refraction. 

Interference  Colors. —  Very  low  ist  order,  dark  gray,  etc. 

Alteration  :     Quite  frequent  to  fibrous  or  granular  zeolites. 


Fig.  36.  —  Leucite,  showing  complicated,  interpenetration  twinning  between  crossed 
nicols. 

Distinguished  from  :     Analcite  —  see  under  analcite. 

Remarks  :     Almost  entirely  confined  to  younger  eruptive  rocks,  phonolite,  tephrite 
and  other  leucite  rocks  and  their  tuffs.      Often  found  with  plagioclase,  nephelite,  augite, 


54 


CHARACTERS  OF  MINERALS. 


etc.  It  is  more  or  less  attacked  by  hot  hydrochloric  acid.  H.,  5.5  to  6.  .Sp.  gr.,  2.4 
to  2.5.  The  isolation  of  leucite  from  rock  powder  can  be  better  accomplished  by 
specific  gravity  than  by  chemical  methods. 

ANALCITE. 

Isotropic.  Isometric 

Composition  :  Xa.\lSi.,0,.  +  H.,0. 

Usual  Appearance  in  Sections  :  Secondary  colorless  grains,  with  no  very  charac- 
teristic microstructure  or  properties.  Cleavage  parallel  to  cube  (  oiox)  ,  loo)  usually 
seen.  Index  of  refraction  low  («  ^1.488),  hence  rather  rough  surface.  Between 
crossed  nicols  may  show  optical  anomalies,  but  not  so  marked  as  in  garnet. 

Distinguished  from  :  Leucite,  Sodalite  and  Nephelite.  These  minerals  are 
most  easily  confused  with  analcite  and  resource  must  be  had  to  chemical  tests,  detection 
of  optical  anomalies,  gelatinization  test  or  turbidity  by  heating. 

Remarks  :  Only  occurs  as  secondary  product  (commonly  from  nephelite  or  leucite) 
in  alkali-rich  eruptive  rocks.  Gelatinizes  with  hydrochloric  acid,  and  becomes  turbid 
by  heating.      H.,  5.5.      Sp.  gr.,  2.25. 

SODALITE   GROUP. 
Sodalite,  Haiiynite  (Haiiyne)  and  Noselite(Wosean). 


Isotropic. 
Composition- 


Isometric 


Sodalite,  sNaAlSiO^  +  NaCl. 

Haiiynite,  2(Ka2Ca)Al2(Si(  )J,  +  (Na.,Ca)SO^. 

Xoselite,  2Na.^Al2Si208  +  NajSO^. 
Usual  Appearance  in  Sections:  Dodecahedral  rounded  crystals  or  (S)  irregular 
grains.      Colorless,  yellowish,  greenish  to  deep  blue.      Refractive  index  low  \ii  =  1. 483 
^  S )  —  1 .  503  (  H  )  ] ,  hence  the  surface  appears  rather  rough  in  sodalite  and  slightly  rough 


Fig.  37. — Haiiynite,    showing    dark    centre    and    border,    in   nepelinite. 
Cohen. ) 


( From 


in  haiiynite.  Inclusions,  abundant  and  characteristic,  often  rod-like  and  arranged  regu- 
larly, making  section  translucent  and  especially  dark  at  border.  Fig.  37.  Dodecahedral 
cleavage  sometimes  seen.      Optical  anomalies  may  occur. 

Alteration  :  Takes  place  easily  to  aggregates  of  natrolite,  other  zeolites,  mica,  etc. 


R  UTILE. 


55 


Distinguished  from : 

{a)  One  another  only  by  chemical  tests.  Gelatinization  test  with  hydrochloric 
acid  will  show  in  addition  to  jelly,  salt  crystals  for  (S),  abundant  gypsum  crystals 
(CaSO^  +  2H20)  for  (H)  and  few  if  any  gypsum  crystals  (absence  of  Ca)  for  (N), 
(H)  and  (N)  turn  blue  when  heated,  but  test  will  not  work  if  minerals  are  decomposed. 
When  treated  with  hydrofluoric  acid  and  nitrate  of  silver  the  black  sulphide  of  silver 
will  show  on  (H)  and  (N),  but  the  white  chloride  on  (S). 

{d)   Nephelite  (Elpeolite)  by  being  isotropic. 

(f )   Analcite  by  no  turbidity  when  heated. 

Remarks  :  These  minerals  are  found  in  the  basic,  soda-rich  rocks.  (S)  in  elseolite 
syenite  also  in  trachyte  and  phonolite.  (H)  and  (X)  common  in  phonolite  and  leucite- 
porphyry.      H.,  5.5  to  6.      Sp.  gr.,  2.3. 

PEROVSKITE,  Perofskite. 

Isotropic.  Isometric. 

Composition  :  CaTiOj. 

Usual  Appearance  in  Sections  :  Microscopic,  octahedral  crystals  or  larger  grains, 
pale  brownish  in  color  and  not  very  transparent,  darker  colored  in  the  larger  grains. 
In  reflected  light  grains  appear  yellowish  with  adamantine  lustre.  Refractive  index 
very  high  («^2.38),  hence  re/ief  very  strong. 

Between  crossed  nicoh,  the  little  crystals  generally  appear  optically  normal  and  re- 
main dark  ;  but  the  larger  crystals  may  show  a  complicated  penetration  twinning. 

Distinguished  from:  Garnet  (melanite)  and  Spinel  (picotite)  by  much  higher 
refractive  index  and  reaction  for  titanium  and  by  zonal  coloration  of  melanite.  When 
opaque  it  might  be  mistaken  for  the  iron  ores,  but  has  no  metallic  lustre. 

Remarks  :  Found  in  the  younger  basic  eruptive  rocks,  especially  melilite-basalt. 
Commonly  associated  with  the  iron  ores,  nephelite,  augite  and  chrysolite.  Insoluble  in 
hydrochloric  acid.      H.,  5.5.     Sp.gr.,  4.1. 

RUTILE. 

Anisotropic.  Uniaxial.  Tetragonal. 

Composition  :  TiO,.  «'=c.  Elongation  ||  c^ 

Usual  Appearance  in  Sections :  Sharp,  elongated,  prismatic  crystals  when  micro- 
scopic,  but  granular  when  the  individuals  are  large.      Grains  may  be  almost  opaque, 


Rutile  twins. 


Fig. 


•  Twin  plane  ( loi 


Fig.  39.  — Twin  plane  (301' 


with  adamantine  lustre  by  reflected  light.      Knee-  or  heart-shaped  twins.  Figs.  38  and 
39,  common  in  the  smaller  crystals    the   larger  individuals   may  also  show  geniculated 


56  CHARACTERS  OF  M/XERALS. 

twinning.  Small  crystals  sometimes  form  net-shaped  groups  (sagenite),  by  crossing 
one  another  at  angles  of  60°.  Pleochroic  halos  may  surround  crystals.  Color,  yellowish 
to  reddish  brown.  Index  of  refraction  very  high  (;?'' =^  2.712),  hence  ;-<V?'dy  marked 
and  surface  very  rough.  Prismatic  cleavage  ])resent  in  larger  individuals,  not  observed 
in  microscopic  crystals.  Pleochroism  and  strong  absorption  may  be  noticed,  especially 
in  the  larger  grains,  but  may  fail  entirely. 

Crossed  Nicols  :  Double  refraction  zw-r  strong  [-, —  0=^0.287).  Interference 
•colors*  very  high  order,  only  seen  in  the  microlitic  crystals  which  do  not  appear  dark 
due  to  total  reflection  ;  in  other  cases  may  not  show  at  all.  Extinction  parallel  to 
prisms.      In  converi^ent  lig/it  o'piicsil  character  (-p). 

Alteration  :  May  take  place  to  a  white  or  yellowish,  fibrous  or  granular  substance, 
strongly  refracting,  and  similar  to  the  alteration  product  of  ilmenite.  May  be  sur- 
rounded by  grains  of  titanite. 

Distinguished  from  :  The  Opaque  Ores  by  adamantine  lustre  with  reflected  light ; 
Zircon  and  Cassiterite  in  concentrates  by  chemical  tests.  May  not  be  possible  to 
distinguish  from  cassiterite  in  sections. 

Remarks  :  P'ound  in  the  metamorphic  schists,  amphibolites,  slates,  contact  and 
fragmentary  rocks,  etc. ;  also  as  inclusions  in  quartz  and  mica.  Especially  common  as 
a  secondary  product  of  titaniferous  hornblende  and  biotite.  The  "sagenite"  webs  of 
the  decomposed  micas  are  probably  secondary.  Rutile  is  insoluble  in  hydrochloric  acid. 
H.,  6  to  6.5.  Sp.  gr.,  4.2.  It  is  easily  separated  from  rock  powder  by  its  insolubility 
in  acid  and  its  high  sp.  gr. 

ZIRCON. 

Anisotropic.  Uniaxial.  Tetragonal. 

Composition  :  ZrSiO^.  c  =  c.  Elongation  ||  c'. 

Usual  Appearance  in  Sections  :  Small,  short  prismatic  crystals, 
Fig.  33  a,  and  grains.  Shell-like  (zonal)  structure  may  be 
noticed.  When  enclosed  in  black-mica,  hornblende,  cordierite, 
etc.,  often  surrounded  by  characteristic  pleochroic  halo. 

Color. —  Colorless,  rarely  pale  brownish. 

I)idex  of  Refraction. —  n'  =  1.95,  hence  relif  very  high  and 
surface  rough. 

Polarized  Light : 

Pleochroism. —  Not  usually  noticeable. 

Crossed  Nicols  : 

Double  Refraction. —  Very  strong  (j-  —  «  =  0.062). 

Intcrfcrcjice  Colors. —  Very  high  (4th)  order,  minute  crystals 
show  brilliant  colors. 

Extinction. — As  zircon  is  unia.xial,  basal  sections  remain  dark 
during  rotation  of  stage.  In  all  other  sections  extinction  is  paral- 
lel to  c. 

*The  interference  colors  of  all  minerals  here  recorded  are  those  given  by  sections 
0.03  WW.  in  thickness  (very  thin  sections). 


SC A  POLITE    GROUP.  57 

Convergent  I^ight:  Basal  sections,  which  are  large  enough  to 
give  interference  figures,  show  several  rings  in  addition  to  dark 
cross.      Optical  character  (  +  ). 

Alteration  :   Ver)-  rarely  takes  place. 

Distinguished  from  : 

[a)  Apatite. —  By  much  higher  relief  and  stronger  double 
refraction. 

(d)  TiTAXiTE. —  By  uniaxial  character. 

(c)   RuTiLE. —  See  under  the  latter. 

Easil}'  confused  with  xenotime,  which,  however,  has  higher 
interference  colors  and  more  distinct  pleochroism  ;  but  chemical 
tests  may  be  necessar\'. 

Remarks  :  Found  widely  distributed  but  not  in  quantity  in  eruptive  and  metamor- 
phic  rocks.  Occurs  in  granite,  syenite,  diorite,  gabbro,  gneiss,  etc.  It  is  one  of  the 
oldest  constituents  of  the  rocks  in  which  it  occurs,  and  may  often  be  found  as  inclusions 
in  the  ferro-magnesium  minerals.  Zircon  is  insoluble  in  hydrochloric  acid.  H.,  y^S- 
Sp.  gr. ,  4.5  to  4.7.  It  can  easily  be  separated  from  rock  powder  on  account  of  its 
high  sp.  gr.,  insolubility  in  acid  and  non-magnetic  properties.  The  crystals  can  then 
be  examined  separately,  or  chemical  tests  made  to  prove  the  presence  of  Zr. 

SCAPOLITE  GROUP, 

Wernerite,  etc. 

Anisotropic.  Uniaxial.  Tetragonal 

Composition  :  Silicates  of  Ca,  Al  and  Xa.  c  =  a.  Elongation  ||  a^. 

Usual  Appearance  in  Sections  :  Colorless  grains,  lath-like  individuals  or  prisms 
(dipyre,  in  contact  metamorphic  limestone).  Index  of  refraction  («'':=  1.55^  to  1. 582) 
about  the  same  as  quartz,  hence  usually  no  relief  and  surface  smooth.  Cleavage  dis- 
tinct, parallel  to  square  prism.  Inclusions  (carbonaceous) may  be  abundant  in  contact 
rocks. 

Crossed  Nicols  :  Double  refraction  usually  strong,  but  varies  (}'  —  a  =0.013  to 
0.036),  increases  with  the  Ca  percentage.  Interference  colors  upper  1st  or  2d  order, 
more  brilliant  than  those  of  most  of  the  colorless  minerals.  Basal  sections  (showing 
cleavages  intersecting  at  90°)  isotropic.  Extinction  parallel  in  longitudinal  sections. 
In  convergent  light  basal  sections  show  distinct  uniaxial  interference  figure  ;  optical 
character  ( — ). 

Alteration  :   Takes  place  easily  to  a  fibrous  substance  or  to  kaolin,  muscovite,  etc. 

Distinguished  from  : 

((?)  Feldspars  (not  showing  twinning)  and  Iolite  (Cordierite)  by  uniaxial  char- 
acter, cleavage  and  higher  order  interference  colors. 

(/^)  Quartz  by  cleavage,  higher  order  interference  colors  and  optical  character  ; 
quartz  is  (+)• 

(f)  Apatite  (in  grains)  by  lower  index  of  refraction,  cleavage  and  higher  order 
interference  colors. 

Remarks:  Found  especially  in  metamorphosed  diabases  and  gabbros  (Norwegian)  ; 
also  in  gneisses,  crystalline  schists,  metamorphosed  limestones,  etc.     Dipyre  occurs  in 


5 8  c  IfARA  CTERS  OF  MINERALS. 

contact  zones  of  limestones  and  schists,  w lieie  it  might  t)e  confused  with  andalusite,  hut 
cross-sections  show  uniaxial  character.  The  minerals  of  this  group  are  more  or  less 
soluble  in  hydrochloric  acid.  When  the  scapolite  contains  CI,  the  following  test  can 
be  made  on  fresh  material.  Treat  with  a  solution  of  silver  nitrate  in  hydrofluoric  acid 
and  the  jelly  will  be  impregnated  with  chloride  of  silver  which  will  turn  brown.  H., 
5.5.     Sp.  gr.,  2.68. 

VESUVIANITE,  Idocrase. 

Anisotroi'IC.  Uniaxial.  Tetragonal. 

Composition:  Ca.,^Alj2(OH)j|,Si2(,0-..  ?  f-  =  a.  Elongation  ||  a'. 

Usual  Appearance  in  Sections  :  Grains  or  prismatic  crystals.  Almost  colorless 
to  reddish  (when  containing  Mn).  Index  of  refraction  high  («'  =  1. 721 ),  hence  relief 
marked  and  surface  rough.  Cleavage  imperfect,  parallel  to  prism.  Pleochroism  gen- 
erally very  faint. 

Crossed  Nicols  :  Double  refraction  very  weak  (y  —  a  _=  0.002),  may  vary  in  differ- 
ent portions  of  the  same  crystal  (optical  anomalies  due  to  the  mineral  being  at  times  a 
mixture  of  isomorphous  individuals).  Interference  colors  very  low  ist  order,  dark  gray, 
etc.,  may  often  appear  zonal.  Basal  sections  isotropic  when  normal,  but  may  show 
division  into  biaxial  portions.  Extinction  parallel  in  sections  elongated  ||  c  axis.  In 
convergent  light  basal  sections  show  a  faint  cross  when  normal  ;  optical  character  gen- 
erally (  — ). 

Alteration  :  Xot  known  in  rock -making  vesuvianile. 

Distinguished  from  :  Epidote  (Pistacite)  by  the  very  low  order  interference  colors. 
Corundum  by  weaker  double  refraction.  Garnet  (Grossularite),  Zoisite,  and  Apa- 
tite may  be  easily  confused  with  this  mineral  and  hard  to  distinguish  from  it. 

Remarks  :  Found  in  limestones,  that  have  undergone  alteration  by  contact  with 
igneous  rocks,  and  in  metamorphic  schists.  Also  may  occur  in  dense  (nephrite-like) 
aggregates  in  serpentine.  It  isinsoluble  in  hydrochloric  acid.  H.,  6.5.  Sp.  gr.,  3.3 
to  3.8. 

MELILITE. 
Anisotropic.  Uniaxial.  Tetragonal. 

Composition  :  CajjAl^SiyOjg,  with  at  times  also  Na,  Mg  and  Fe. 
c  ■=-.  a.  Elongation  ||  z' . 

Usual  Appearance  in  Sections:  Almost  colorless,  tabular  (  ||  base)  crystals  or 
irregular  grains  or  shreds.  Sections  very  commonly  lath-shaped,  and  often  character- 
ized by  the  peculiar  "peg-structure,"  *  the  lines  or  markings  being  |1  /  (  -J-  elongation 
of  the  section).  Index  of  refraction  («'  =  1.630)  higher  than  that  of  the  other  associ- 
ated colorless  materials,  hence  relief  rather  marked.  Cleavage,  parallel  to  base,  very 
imperfect. 

Crossed  Nicols  :  Double  refraction  very  weak  (>'  —  n^  0.003),  ^"^^  diminishes 
with  a  decrease  of  Al.  Interference  colors  the  lower  1st  order,  grays,  etc.;  anomalous 
interference  colors  may  show.  Extinction  parallel  to  cleavage  or  the  peculiar  mark- 
ings or  lines.      Optical  character  usually  (  —  ),  but  when  poor  in  Al  (  -f  ). 

Alteration :  Takes  place  frequently  to  a  fibrous  aggregate. 

Distinguished  from:  Nephelite  and  Feldspar  by  higher  relief,  shape,  "peg- 
structure"  and  usual  dull  appearance  with  reflected  light. 

Remarks  :  Abundant  in  the  leucite  and  nephelite  rocks  (associated  with  these  min- 
erals and  with  augite,  perovskite  and  chrysolite),  and  takes  the  place  of  a  feldspar  in 
the  melilite-basalt.     It  gelatinises  easily  with  hydrochloric  acid.      H.,  5.     Sp.  gr.,  2.9. 

*  Iddings'   Kosenbusch,  p.   160. 


ILMEXITE.  59 

GRAPHITE. 

Hexagonal. 
Composition  :  C. 

Usual  Appearance  in  Sections  :  Minute  particles,  or  flakes  and  grains  of  irregular 
shape,  seldom  crystallized. 

Opaqite,  and  by  reflected  light,  black  with  metallic  lustre. 

Distinguished  from :  the  similarly  appearing  ores  by  its  insolubility  in  acids  and 
the  possibility  of  making  it  disappear  by  heating. 

Remarks  :  Graphite  is  widely  distributed  in  the  oldest  rock  formations,  especially 
in  the  schists.  It  is  often  associated  with  rutile  and  the  iron  oxides.  Graphite  is  not 
acted  on  by  acids.  H.,  I  to  2.  Sp.  gr.,  2.09  to  2.25.  It  is  burnt  with  great  difficulty 
in  thin  sections  on  platinum  foil  ;  but  this  test  may  vary,  in  many  cases  the  graphite 
(when  in  bladed  flakes)  not  being  consumed  even  after  long  heating.  When  heated  it 
may  expand  into  worm-like  forms. 

Carbonaceous  Matter.  —  Occurs  in  opaque,  grayish-black  particles  having  no  lustre  ; 
and  is  found  finely  disseminated,  sometimes  in  larger  aggregations,  in  clay  slates,  lime- 
stones, etc. 

HEMATITE. 

Hexagonal. 
Composition  :  I^.,©,. 

Usual  Appearance  in  Sections  :  Irregular  scales,  minute  grains  or  earthy.  Dis- 
tinct crystalline  forms  not  often  observed  in  rocks. 

Opaque,  and  by  reflected  light,  black  with  metallic  lustre,  or  red  without  lustre. 
May  also  be  transparent  in  red  tints.      No  marked  pleochroism  observed. 

Remarks  :  Founds  widely  distributed  in  acid  eruptive  rocks,  crystalline  schists,  etc. 
Also  as  inclusions  in  minerals,  and  as  a  red  pigment  in  many  rocks.  It  is  insoluble  in 
hydrochloric  acid,  and  non-magnetic,  unless  attached  to  grains  of  magnetite.  H.,  5.5 
to  6.5.      Sp.  gr.,  4.9  to  5.3. 

ILMENITE,  Menaccanite. 
Hexagox.\l. 

Composition  :  (FeTi).,03. 

Usual  Appearance  in  Sections  :  Irregular  masses,  without  crys- 
tallographic  outline,  rhombohedral  crystals,  or  skeleton  growths. 
Also  in  brownish,  translucent  mica-like  forms. 

Opaque,  and  by  reflected  light,  iron-black  with  metallic  lustre. 

When  trmisliicent :  pleochroism  brown  to  yellow  ;  double  re- 
fraction not  very  strong  ;  optically  ( — ). 

Alteration  :  Often  takes  place  to  a  whitish,  strongly  refracting, 
substance  only  slightly  transparent,  called  leticoxcnc.  This  altera- 
tion product  frequently  develops  along  definite  rhombohedral  di- 
rections, Fig.  33  c.  Also  a  change  to  titanite  or  rutile  may  occur, 
or  the  ilmenite  may  be  surrounded  by  these  minerals. 

Distinguished  from  :  Magnetite  and  Hematite. —  By  whitish, 


6o  CHAR  ALTERS  OF  MINERALS. 

strongly  refracting  decomposition  product.      At  times  the  distinc- 
tion may  be  very  difficult. 

Remarks  :  Ilmeiiite  occurs  principally  in  the  soda-rich  and  basic  eruptive  rocks. 
The  mica-like  form  is  limited  to  the  porphyritic  eruptives.  The  brown  pigment  in  the 
plagioclase  of  certain  gabbros  may  be  ilmenite.  It  is  attacked  slowly  by  hot  hydro- 
chloric acid,  and  the  solution  when  heated  with  tin  becomes  violet.  Pure  ilmenite  is 
indifferent  towards  the  magnet,  hence  strong  magnetic  properties  would  indicate  a  mix- 
ture with  magnetite.      H.,  5  to  6.      Sp.  gr.,  4.5  to  5. 

CORUNDUM. 

Anisotropic.  Uniaxial.  Hexagonal. 

Composition  :  AL^O.j.  c  =  a- 

Usual  Appearance  in  Sections  :  Pyramidal  or  prismatic  crystals,  grains  or  basal 
plates.  Zonal  structure  or  twinning  may  be  noticed.  Colorless  or  with  patches  of 
blue.  Index  of  refraction  high  («''=  1.766),  hence  relief  V4c\\  marked  and  surface 
very  rough.  Rhombohedral  cleavage  may  show  in  larger  individuals.  Pleochroism 
only  marked  when  color  is  deep. 

Crossed  Nichols  :  Double  refraction  weak  (y  — a  =0.009),  ''ke  quartz.  Interfer- 
•ence  colors  middle  1st  order,  white  to  yellow.  Extinction  parallel  in  elongated  sec- 
tions. Optical  anomalies  very  rarely  noticed  in  microscopic  individuals.  In  convergent 
/?^/;^  basal  sections  show  a  rather  indistinct  cross  ;  optical  character  (  —  ). 

Distinguished  from : 

(rt)   Apatite  and  VESUViANiTEby  brighter  interference  colors. 

((J)  Tourmaline  (light  colored)  by  not  having  such  strong  absorption. 

(f)   Cyanite  by  uniaxial  character. 

Corundum  may  need  to  be  isolated  from  the  rock  in  order  to  be  determined  with 
certainty. 

Remarks  :  Found  in  contact  metamorphic  rocks,  eruptive  rocks,  granular  lime- 
stones, etc.  It  is  insoluble  in  hydrochloric  acid.  When  rock-sections  are  ground  with 
€mery,  care  must  be  taken  not  to  confuse  grains  of  emery  with  corundum  in  the  rock. 
H.,  9.      Sp.  gr.,  3.9  to  4. 

QUARTZ. 

Anisotropic.  Uniaxial.  Hexagonal. 

Composition  :  SiOj.  t'  =  c. 

Usual  Appearance  in  Sections  :  Allotriomorphic  in  the  grani- 
toid rocks,  when  apparently  the  last  mineral  to  form,  Fig.  5. 
More  or  less  chemically  corroded  pyramidal  crystals  (with  cross- 
sections  six-sided  or  rhombic  with  an  angle  of  about  100°)  in  the 
porphyritic  rocks.  Rounded  or  angular  grains  in  the  "  clastic  " 
rocks  ;  granular  mosaic  in  crystalline  schists  and  contact  rocks. 
Very  rarely  in  distinct  crystals  in  any  rocks.  May  at  times 
be  mutually  interpenetrated  with  an  acid  feldspar  (the  areas  of 
quartz  and  feldspar  extinguishing  as  entire  crystals),  producing 
"  micropegmatitic  "  structure.  Finally  may  appear  as  pseudo- 
morphs  after  other  minerals,  but  may  then  consist  of  some  of  the 
other  forms  of  silica. 


(JUARTZ.  .  6r 

Color. — Colorless,  although  by  reflected  light  it  may  appear 
colored  or  cloudy  if  it  contains  many  inclusions. 

Index  of  Refraction. — n'  =  1.547,  hence  no  relief  and  surface 
smooth. 

Cleavage. — Rarely  noticed,  an  important  fact  in  determining 
quartz.      Quartz  breaks  irregularly. 

Inclusions. — Minute  fluid,  gas  and  mineral  inclusions,  often  in 
irregular  trains,  are  very  characteristic  of  quartz  in  granite  rocks 
and  crystalline  schists.  The  inclusions  are  not  so  abundant  in 
porphyritic  rocks,  but  a  few  glass  inclusions  may  occur,  filling  up 
"  negative  "  crystals  in  the  quartz.  Rutile,  amphibole,  etc.,  may 
occur  as  needle-like  inclusions  in  quartz. 

Polarized  Light : 

Pleochroisni. — None. 

Crossed  Nicols : 

Don  lie  Refraction. — Weak  (y  —  a  =  0.009). 

Interference   Colors. — The  middle    ist  order,  white,   yellow,  etc. 

Extinction. — As  quartz  is  uniaxial,  basal  sections  remain  dark 
during  a  complete  rotation  of  stage.  In  the  other  sections  extinc- 
tion is  not  characteristic,  due  to  the  absence  of  cleavage  and  crys- 
tallographic  outlines.  Thin  sections  do  not  show  circular  polar- 
ization. 

Convergent  Light :  Basal  sections  show  a  dark  cross,  without 
any  rings.      Optical  character  (  +  ). 

Alteration  :  Does  not  take  place,  so  quartz  always  appears  fresh 
and  unweathered  in  sections. 

Distinguished  from  : 

{a)  Saxidine  (in  fresh  grains). — By  use  of  convergent  light. 
Feldspar  is  biaxial,  or  sections  which  appear  uniaxial  are  (  —  ). 

{d)  Nephelite. — By  almost  entire  absence  of  hexagonal  outline, 
stronger  double  refraction,  fresh,  unweathered  appearance  and  (-|-) 
optical  character. 

(c)  loLiTE  (Cordierite),  Scapolite  and  Topaz. — See  under  the 
latter  minerals. 

Quartz  may  be  distinguished  from  all  silicates  by  being  dissolved 
without  residue  in  hydrofluoric  acid. 

Remarks  :  Quartz  occurs  widely  distributed,  as  in  the  great  sandstone  formations. 
It  is  also  a  characteristic  mineral  of  all  acidic  rocks,  being  common  in  granite,  aplite, 
rhyolite,  quartz-porphyry,  quartz-diorite,  dacite,  etc.      Quartz  is  very  brittle  and  hence  i.i 


62  CHARACTERS  OF  MINERALS. 

a  good  indicator  of  the  dynamic  forces  which  have  affected  the  rocks.  It  may  show 
traces  of  mechanical  deformation  by  peripheral  shattering  of  the  larger  grains  or  by 
"  wavy  extinction  "  *  ;  and  also  evidences  of  chemical  corrosion  by  curved  and  looped 
contours.  In  some  diabases  the  quartz  may  be  surrounded  by  a  rim  of  hornblende  or 
augite  needles  (  "quartz  augen  "  ).  "  Cataclastic  "  quartz  may  be  biaxial.  The  "  sec- 
ondary enlargement  "  of  quartz  in  clastic  rocks  may  be  noticed  by  the  deposition  of  silica 
in  crystallographic  orientation  around  the  clastic  grains  f,  the  new  portion  extinguishing 
at  the  same  time  as  the  core.  Quartz  is  not  attacked  by  ordinary  acids.  H.,  7.  Sp. 
gr.,  2.6  to  2.7. 

Chalccdo)iy. — This  v^ariety  of  quartz  has  a  radially  fibrous  struc- 
ture and  shelly  parting.  It  may  form  sphserulites,  central  sections 
through  which  show  a  dark  cross  between  crossed  nicols,  or  line 
cavities  in  rocks. 

The  index  of  refraction  is  a  little  lower  than  for  ordinary  quartz. 
The  optical  character  is  (  —  ),  which  must  be  determined  by  a  mica 
or  gypsum  plate,     c  =  n.      Elongation  ||a'. 

Chalcedony  occurs  in  the  ground  mass  of  very  silicious  porphyritic  rocks,  which  have 
microfelsitic  development ;  and  is  found  as  a  secondary  mineral  in  all  kinds  of  silicate 
rocks. 

TRIDYMITE. 

Usual  Appearance  in  Sections  :  This  form  of  SiOj,  which  is  soluble  in  boiling 
caustic  soda,  appears  in  "tile-like"  aggregates  of  minute  colorless  plates  (pseudohex- 
agonal)and  is  always  secondary.  The  refractive  index  is  extremely  low  {^11'  ^  1.477), 
hence  the  surface  appears  rough. 

Between  crossed  nicols  the  interference  colors  are  very  low  in  order  (}'  —  a  z=z  0.002), 
and  the  tablets  may  show  a  division  into  different  areas  (optical  anomalies).  In  con- 
vergent light  an  indistinct  biaxial  figure  is  generally  seen. 

Remarks  :  Chiefly  a  volcanic  mineral,  found  in  rhyolite,  trachyte  and  andesite. 
Commonly  associated  with  opal  and  chalcedony. 

CALCITE. 

Anisotropic.  Uniaxial.  Hexagonal. 

Composition  :  CaCOg.  Ca  may  be  replaced  by  small  quantities 
of  Mg,  Fe,  Mn,  etc.  c  =  a. 

Usual  Appearance  in  Sections  :  Grains  and  aggregates.  May 
be  fibrous  or  oolitic.      Only  in  cry.stals  in  certain  rocks.;}; 

Tzviniiiitg.  —  Polysynthetic,  parallel  to  —  i'<R(oii2).  \'ery 
common  in  crystalline  limestones,  and  may  have  been  produced  by 
pressure.  Shows  itself  between  crossed  nicols  as  a  series  of  light 
and  dark  bands,  P^ig.  40,  about  parallel  to  longer  diagonal  of 
cleavage  rhombs.      Twin  lamellae  may  be  so  fine  as  to  produce 


*  See  page  30, 

t  R.  D.  Irving,  A711.  Joitr.  Sci.,  June,    1S83. 

\  Granites  of  the  central  Alps,  where  the  calcite  crystals  are  intergrown  with  quartz 


CALCITE.  63 

fibrous  appearance.  When  the  composition  face  of  the  twins  is 
obHque  to  the  face  of  the  section,  interference  colors  can  be  seen 
without  the  analyzer. 

Color.  —  Colorless  when  pure,  but  may  appear  colored  by  trans- 
mitted light,  due  to  organic  pigments. 


Fig.  40.  — Calcite,  crossed  twin  lamella-,  in  granular  limestone.      (From  Cohen.) 

Index  of  Refraction.  —  ;/'  =  1. 60 1,  hence  fairh'  marked  relief  3.116. 
rather  rough  surface.  Due  to  the  great  variation  in  refractive 
indices  of  the  two  rays  (a  =  1.487,  y  =  1-659),  with  polarized  light 
the  surface  will  appear  either  quite  smooth  or  rather  rough,  de- 
pending upon  which  vibration  direction  lies  over  the  plane  of  the 
polarizer.  This  marked  variation  in  appearance  ser\'es  as  a  good 
test  for  calcite. 

Cleavage.  —  Parallel  to  rhombohedron  (R,  ion),  appearing  in 
thin  sections  as  many  sharp  cracks,  whose  angles  of  intersection 
depend  on  the  position  of  the  section.  Fig.  41.  Newton's  colors 
may  be  seen  along  cleavage  cracks. 

Polarized  Light : 

PleocJiroism.  —  Xo  change  of  color  obser\ed,  but  absorption  of 
the  ordinary  ray  can  be  noticed  when  section  is  not  too  thin. 

Crossed  Nicols : 

Double  Refraction.  —  Very  strong  (;-  —  o.  =  o.  172). 

Interference  Colors.  —  Pale,  iridescent  colors  of  very  high  order. 

E.xtinction.  —  As  calcite  is  uniaxial,  basal  sections  remain  dark 
during  rotation.  Extinction  with  respect  to  the  cleavage  cracks 
varies  with  the  position  of  the  section. 

Convergent  Light:   Basal  sections,  even  when  very  thin,  show 


64  C/fARACTERS  OF  MIXERALS. 

distinct  interference  figure,   with  cross  and  rings.      Optical  char- 
acter (  — ). 

Distinguished  from  : 

(rt)   Other  Caki'.dnates.  —  By  ease  with  which  it  is  attacked  by 
cold  dilute  acids,  test  can  be  made  on  sHde  after  removing  cover. 
[1))  Magnesium-bearixg  Calcite.  —  By  micro-chemical  tests. 
{c)  TiTAMTE  (Sphene).  — See  under  the  latter. 


'Fig.  41.  —  Calcite,  section  parallel  to  face  of  rhoiiibohedroti,  showing  rhombohedral 
cleavage.      (  From  Cohen. ) 

Re.marks  :  Calcite  is  very  widely  distributed,  in  addition  to  the  e.xtensive  sedi- 
mentary limestone  deposits.  Common  limestone  consists  of  dense  aggregates  of  crystal- 
line grains.  Calcite  is  often  a  secondary  product  of  the  lime-bearing  silicates  in  the 
more  basic  eruptive  rocks,  generally  in  such  cases  not  showing  good  cleavage  (Wein- 
schenk).  Pseudomorphs  of  calcite  after  olivine  are  noteworthy.  Coarse  aggregates 
of  calcite  occur  in  the  crystalline  schists  and  contact  rocks.  Calcite  is  exceedingly 
plastic  to  pressure  and  mechanical  deformation  may  be  recognized  by  curving  of  the 
cleavage  cracks,  crumpling  of  the  twin  lamellae  and  "wavy"  extinction.  Calcite  is 
easily  attacked  and  completely  dissolved  with  effervescence  by  cold  dilute  acids.  H.,  3. 
Sp.  gr.,  2.72. 

DOLOMITE. 

Anisotropic.  Uniaxial.  Hexagonal. 

Composition  :  (Ca.Mg)Co3,  when  pure  CaO  =  30.4,  MgO  =21.7, 
CO.,  =  47.8.  Proportions  of  Mg  and  Ca  vary,  and  Fe  and  Mn  also 
occur.  c  =  a. 

Usual  Appearance  in  Sections  :  In  rocks  chiefly  as  crystals,  even 
dense  homogeneous  aggregates  showing  tendency  towards  crystal- 
line boundaries  (saccharoidal  structure).  Crystals  almost  always 
unit  rhombohedron  (R,  loTi)  with  tendency  to  curved  surfaces. 

Index  of  Refraction.  —  ;/  =  1.622,  a  little  higher  than  that  of 


A  PA  TITE.  65 

calcite.      For  variation    in   appearance    of  surface  with    polarized 
light,  see  under  calcite. 

The  microscopic  characters  are  similar  to  those  of  calcite,  from 
which  it  may  be  distiiiguislicd  by  not  being  so  easily  attacked  by 
cold  dilute  acid  (test  can  be  made  on  slide  with  cover  off),  by  ten- 
dency towards  crystalline  boundaries,  by  absence  of  twin  lamellae 
(or  when  present  parallel  to  —  2/?(o2  2i),  hence  about  parallel  to 
shorter  diagonals  of  cleavage  rhombs),  and  by  micro-chemical 
tests.      The  distinction  at  times  may  be  very  difficult. 

Remarks  :  Occurs  in  sedimentary  formations  and  as  crystals  in  limestone  and  other 
rocks.  In  certain  rocks  the  dolomite  crystals  may  not  have  a  very  good  "  bond,"  and 
a  "  drusy  "  structure  may  also  be  characteristic  of  the  spaces  between  dolomite  crystals 
in  rocks.  Only  slightly  attached  by  cold  dilute  acids,  but  if  acid  is  heated  it  dissolves 
easily  with  effervescence. 

APATITE. 

Anisotropic.  Uniaxial.  Hexagonal. 

Composition:  Ca,(Cl.F)(PO,)3.  r=  a.  Elongation  ||  a'. 
Usual    Appearance    in    Sections :     Minute,    slender,    hexagonal 

prisms,     cross-sections     having     regular     hexagonal     boundaries, 

needles,  and  grains.      Figs.   14  a  and  42. 


Fig.  42.  —Apatite,  showing  cross  fracture,  in  nepheline-basalt.      (From  Cohen.) 

Color.  —  Generally  colorless,  seldom  bluish   or  brownish  (only 
in  eruptive  rocks). 

Index  of  Refraction. — //' =  1.635,    hence  relief  more  marked 
than  that  of  the  colorless  associated  minerals. 

Cleavage.  —  Seldom  observed  microscopically. 

Parting.  —  Long  columnar  crystals  generally  show  a  transverse 
jointing,  so  that  the  pieces  may  be  more  or  less  separated. 
5 


66  CHARACTERS  OF  MIXERALS. 

I)iclitsio)is.  —  (ias  and  fluid  ma}-  be  present. 
Polarized  Light : 

Plcochroism.  —  None  shown  b\-  the  colorless  crystals,  the  colored 
crystals  show  stronger  absorption  parallel  to  c . 

Crossed  Nicols : 

Double  Refraction.  —  Weak  ( j-  —  (/.  =  0.003). 

Interference  Colors. —  The  lower  first  order,  generall}'  grayish- 
blue  or  white. 

Extinction. — As  apatite  is  uniaxial,  basal  sections  remain  dark 
during  rotation  of  stage.  In  all  other  sections  extinction  is  parallel 
to  c  axis. 

Convergent  l,ight :  Basal  sections  show  a  cross,  without  rings. 
Optical  character  (  —  ). 

Alteration  :  Does  not  usually  take  place,  apatite  being  found 
perfectly  fresh  in  decomposed  rocks,  which  is  quite  remarkable 
considering  its  easy  solubility  in  acids. 

Distinguished  from  : 

{a)  SiLLiMAXiTE  and  Tremolite.  —  By  weak  double  refraction 
and  elongation  ||  a'. 

(p)  Nephelite.  —  By  being  relatively  much  smaller  and  longer 
than  the  nephelite  crystals,  which  are  often  decomposed.  Also  by 
higher  relief  and  negative  results  with  gelatinization  test. 

(<:)  ZiRCOX.  —  By  lower  relief  and  much  weaker  double  re- 
fraction. 

{d)  Feldspars  (when  granular  and  undecomposed). —  B}'  higher 
relief  and  uniaxial  interference  figure. 

[e)  Vesuvianite  and  Zoisite.  —  May  be  only  possible  by 
chemical  tests. 

(/)    CoRUXDU.M.  —  See  under  the  latter. 

Remarks  :  Found  in  most  igneous  rocks  and  crystalline  schists.  In  the  eruptive 
rocks  it  appears  as  one  of  the  oldest  secretions  from  the  magma,  and  hence  is  often  found 
as  inclusions  in  other  minerals,  especially  biotite,  hornblende,  etc.  Apatite  is  easily 
soluble  in  hydrochloric  and  nitric  acids.  H.,  4.5  to  5.  Sp.  gr.,  3.19.  On  account  of 
its  high  sp.  gr. ,  apatite,  in  rock  powder,  comes  down  in  heavy  solutions  with  the 
metallic  minerals,  and  can  be  separated  from  them  by  the  use  of  a  magnet.  This  resi- 
due can  also  be  tested  for  phosphorus  in  the  wet  way  with  ammonium  molybdate. 


XEPHELITE. 


67 


•I 

"i 

!U 

1" 

Fig.  43.  —  Nephelite 
showing  zonal  inclusions. 
Reinisch. ) 


sections, 
(  From 


NEPHELITE,*  Nepheline,  Elaeolite. 

Anisotropic.  Uniaxial.  Hexagonal. 

Composition  ;  /NaAlSiO^  +  NaAl  (SiOg)^,  with  partial  replace- 
ment of  Na  by  K  or  Ca.  c  =  a. 

Usual  Appearance  in  Sections  :    Nephelite  in  short  hexagonal 
prisms  and   grains  in   the  younger  volcanic  rocks,  hence  sections 
rectangular    or    hexagonal,    Fig. 
43  ;  elaeolite  allotriomorphic  in  the 
older  plutonic  rocks. 

Color. —  Colorless. 

Index  of  Refraction.  —  ;/  = 
1. 54 1,  hence  no  relief  and  sur- 
face smooth. 

Cleavage.  —  Imperfect,   parallel 
to   prism    (00  P,    10 10)   and    base 
(0    P,    0001).      More    marked    in 
elaeolite  than  in  nephelite,  especially  when  decomposition  has  com- 
menced. 

Inclusions. —  Microscopic  needles  of  augite,  etc.,  also  fluid  and 
gas.  Mostly  in  zones.  P^lsolite  may  be  much  clouded  by  inclu- 
sions and  alteration  products. 

Polarized  Light  : 

PleocJiroisni. —  None. 

Crossed  Nicols  : 

Double  Ref -action. —  Very  weak  (j'  —  a  =  0.004),  may  only  be 
detected  by  using  a  test-plate. 

Interference  Colors  —  The  lower  first  order,  grayish- white,  etc., 
a  little  lower  than  the  feldspar  colors. 

Extinction. —  As  the  mineral  is  uniaxial,  basal  sections  remain 
dark  during  rotation  of  stage.  In  all  other  sections  extinction 
takes  place  and  is  parallel  to  cleavage  lines  when  these  appear. 

Convergent  Light:  Basal  sections  show  a  broad  cross,  with- 
out rings.      Optical  character  ( — ). 

Alteration  :  Takes  place  easily  to  fibrous  zeolites  (natrolite),  or 
in  certain  rocks  to  mica. 


*  The  minerals  Nephelite,  leucite,  sodalite  (haiiynite  and  noselite)  and  melilite  are 
often  grouped  together  under  the  name  'feldspathoides  " ;  on  account  of  their  relation 
in  rocks  being  equivalent  to  that  of  the  feldspars. 


68 


CHARACTERS  OF  MINERALS. 


Distinguished  from:  Othkr  Minerals  by  gclatinization  test 
and  staining  with  fuchsinc.  When  present  in  smaH  interstitial 
individuals  (as  is  often  the  case  in  basalts)  it  is  very  difficult  to  dis- 
tinguish without  this  test ;  but  it  must  be  remembered  that  other 
minerals,  zeolites,  etc.,  will  also  gelatinize.  Quartz  has  stronger 
double  refraction,  rarely  shows  hexagonal  outline,  is  always  fresh 
and  optically  (  +  ).  Feldspar  is  biaxial  and  often  shows  twinning. 
AxALCiTE — See  under  the  latter. 

Remarks  :  Nephelite  bears  the  same  relation  to  elreolite  as  sanidine  does  to  ortho- 
clase.  It  occurs  only  in  the  younger  volcanic  rocks  ;  with  sandine  in  phonolite,  with 
plagioclase  in  tephrite,  without  feldspar  in  nepheline-basalt,  and  with  leucite  in  leucite- 
basalt.  It  is  not  found  with  primary  quartz.  Elaeolite  occurs  with  orthoclase  in 
elteolite-syenite,  etc.  Nephelite  and  elreolite  frequently  occur  with  the  sodalite  group. 
Nephelite  gelatinizes  with  acids.     H.,  5.5  to  6.     Sp.  gr.,  2.5  to  2.6. 

TOURMALINE,  Shorl. 

Anisotropic.  Uniaxial.  Hexagonal. 

Composition  ;  Uncertain,  R,^B.,(SiO.)^.  R  chiefly  Al,  K,  AIn, 
Ca,  Mg,  Li.  c  =  a.  Elongation  i|  <x' . 

Usual  Appearance  in  Sections  :   Staff-like  individuals,  bunched 


Fig.  44.  — • .  /,  Touniialine,  showing  strong  absorption  at  right  angles  to  direction  of 
elongation  (/*=  plane  of  vibration  of  polarizer).  Qttartzite,  Black  Hills,  D.  v9, 
Tourmaline  in  radiate  aggregate.      Granite,  Cornwall. 

or  in  radiating  aggregates,  Fig.  44  b,  or  prismatic  crystals,  Fig.  44 
a.      Basal  sections  may  be  nine  sided. 

Color. — Varies  greatly,  grayish-blue,  brown  and  green  most 
common.  Li-tourmaline  (rare  in  rocks)  is  colorless.  Zonal 
structure  may  be  indicated  by  differences  in  color. 


TO  UR MA  LINE.  69 

Judex  of  Refraction. — ;/  =  1.635,  hence  relief  is  marked  and 
surface  rough. 

Cleavage. — Not  seen  in  thin  sections,  but  irregular,  transverse 
and  longitudinal  cracks  may  appear. 

Polarized  Light : 

Pleoehroisin. — Distinct,  even  in  light  colored  varieties,  increasing 
with  the  depth  of  color.  The  greatest  absorption  takes  place  at 
right  angles  to  the  direction  of  elongation  of  the  crystal,  Fig.  —  a. 
The  other  minerals  having  this  very  strong  absorption  are  horn- 
blende, dark  colored  mica  (distinguished  by  cleavage  and  lamella 
form)  and  allanite.  Pleochroic  halos  may  be  noticed  surrounding 
inclusions. 

Crossed  Nicols: 

Double  Refractio)i.  —  Quite  strong  {j  —  «  =  0.02  to  0.023). 

luterfcrcuce  Colors.  —  Bright  upper  first  or  lower  second  order. 

Extiuetiou.  —  As  tourmaline  is  uniaxial,  basal  sections  remain 
dark  during  rotation  of  stage.  In  all  other  sections  extinction  is 
parallel  to  ir  axis. 

Convergent  I^ight :  Cross-sections  show  a  sharp  cross.  Optical 
character  (  — ). 

Alteration  :    Does  not  take  place. 

Distinguished  from  : 

(a)  Hornblende.  —  By  absence  of  cleavage,  and  by  the  fact 
that  the  greatest  absorption  takes  place  at  right  angles  to  the 
longitudinal  axis,  while  in  hornblende  it  takes  place  approximately 
parallel  to  the  longitudinal  axis,  or  to  the  cleavage  lines  which  are 
parallel  to  this  axis. 

(d)  Apatite  (when  colored).  —  By  strong  absorption  at  right 
angles  to  longitudinal  axis. 

(r)  Corundum.  —  See  under  the  latter. 

In  some  cases  where  recognition  is  difficult,  chemical  tests,  to 
prove  presence  of  boracic  acid,  must  be  made. 

Remarks  :  The  black  schorl  is  the  only  primary  tourmaline  and  is  found  in  grani- 
toid rocks.  Tourmaline  in  other  rocks  results  from  "  fumarole  "  action  ;  hence  occurs 
in  pegmatite,  tin  and  copper  veins,  clay  deposits,  also  (light  colored)  in  contact  rocks 
and  crystalline  schists.  The  hemimorphic  terminations  may  sometimes  be  noticed. 
Tourmaline  is  not  acted  on  by  acids.  H.,  7  to  7- 5-  Sp.  gr. ,  3  to  3.2.  It  can  be 
separated  from  powdered  rock  by  sp.  gr.  solutions  combined  with  magnetic  methods. 


70  CHARACTERS  OF  M/XERALS. 

ANDALUSITE. 

Amsotroi'IC.  Biaxial.  Orthorhombic. 

Composition:  Al(A10)SiO^.  <=   a.  Elongation  ||  a''. 

Usual  Appearance  in  Sections  :  In  short,  rounded,  prismatic  crystals,  with  almost 
square  cross-section.  Colorless  or  at  times  pale  reddish  and  spotted.  Index  of  refrac- 
tion medium  (;/ ^1.638),  hence  ;r//ty  well  marked  and  surface  rough.  Cleavage, 
parallel  to  almost  square  prism,  may  show.  Pleochroism  only  marked  in  colored  vari- 
eties, being  reddish  ||   'c  (the  direction  of  elongation  or  cleavage).      Carbonaceous  inclus- 


FlG.  45. — Chiastolite,  showing  characteristic  carbonaceous  inclusions.  (From 
Cohen. ) 

ions  are  characteristic,  arranged  as  in  macroscopic  specimens  {Chiastolifc),  Fig.  45. 
Pleochroic  halos  may  surround  inclusions. 

Crossed  Nicols  :  Double  refraction  weak  (y — a  =  0.011).  Interference  colors 
middle  1st  order,  white  to  yellow.  Extinction  in  general  parallel  to  c  axis  in  longi- 
tudinal sections,  symmetrical  in  cross  sections.  In  convergent  light  Ax.  pi.  ||  00  Poo 
(010),  Bx„.  II  c  ;    axial  angle  very  large  {2E~^  180°)  ;  optical  character  ( — ). 

Alteration  :  Often  takes  place  to  dense  aggregate  of  mica,  when  the  pseudomorph 
may  be  hard  to  recognize. 

Distinguished  from : 

(rt)  SiLLlMANlTE  by  much  weaker  double  refraction,  less  elongated  crystals  and  by 
elongation  ||  a^  (Sillimanite  elong.  ||  c^). 

(b)  DiOPSlDE  by  weaker  double  refraction,  rhombic  cross-section  and  parallel  ex- 
tinction in  longitudinal  sections. 

Remarks  :  Very  characteristic  of  metaniorphic  schists  and  of  contact  zones  of  clay 
slates  with  granite,  etc.,  but  not  found  in  rocks  which  have  been  formed  at  great  pressure. 
The  andalusile  grains  may  often  be  arranged  in  divergent  or  finger-like  manner.  May 
also  form  parallel  growths  with  sillimanite.  It  is  insoluble  in  hydrochloric  acid.  H., 
7  to  7.5.     Sp.  gr.,.3.18. 

SILLIMANITE,  Fibrolite. 

Anisotropic.  Biaxial.  Orthorhombic. 

Composition  :  .M(AK))Si(),.  <'  =  c.  Elong.\tion  jj  c'. 

Usual  Appearance  in  Sections  :    Long,  slender,  colorless  prisms  or  needles  ;  often 


TOPAZ. 


71 


in     felt-like    aggregates.       Crystals    often    bent.       Index    of   refraction    rather    high 
{n'  =  1.664),  hence  r<7/V/' marked.     Transverse  fractures  common,  Fig.  46. 

Crossed  Nicols  :  Double  refraction  rather  strong  (} — rt  ^0.021).  Interference 
colors  upper  first  or  lower  second  Order,  red,  purple,  blue,  etc.  Extiudion  parallel  to 
prisms.      Ax.  pi.  ||  00  Poo  (loo),  Bx„.  ||  c ,  2E  =  44°.      Optical  character  (  +  )• 

Distinguished  from  : 

{a)  Apatite  by  higher  order  interference  colors  and  by  elongation  ||  c^  (apatite 
has  elongation  ||  a')- 

{/>)  Tremolite  by  always  parallel  extinction  and  small  size  of  axial  angle. 


Fig.  46. — Sillimanite  aggregate,  showing  cross  fracture,  in  mica-schist.  (From 
Cohen. ) 

((-)   Andalusite,  see  under  the  latter  mineral. 

Remarks  :  Found  especially  in  clay-rich  contact  rocks,  gneisses  and  schists,  often 
occurring  with  iolite  (cordierite).  Crystals  may  appear  in  tjands.  It  is  insoluble  in 
hydrochloric  acid.      H.,  6  to  7.      Sp.  gr.,  3.24. 

TOPAZ. 

Anisotropic.  Biaxial.  Orthorhombic. 

Composition:  A1(A1(0.F.,)  )SiO^.  <-  =  c. 

Usual  Appearance  in  Sections  :  Colorless  crystals  of  short  prismatic  habit,  grains 
or  rod-like  radiating  aggregates.  Index  of  refraction  about  the  same  as  that  of  calcite 
(«'==  1.612-1.632),  hence  ;y//<'/"  medium.  Cleavage  perfect,  parallel  to  base,  but 
does  not  show  as  many  cracks.      Fluid  inclusions  abundant. 

Crossed  Nicols  :  Double  refraction  weak  (y  —  a  =0.008  to  o.oi  i ),  about  the  same 
as  that  of  quartz.  Interference  colors  middle  first  order,  white,  yellow,  etc.  Extinc- 
tion parallel  to  cleavage.  In  convergent  light.  Ax.  pi.  ||  00  Poo  (010),  Bx^.  ||  c ,  axial 
angle  large  (a^?^  70°- 120°)  ;  interference  figure  obtained  from  basal  sections  (/.  e., 
from  sections  showing  no  cleavage)  ;  optical  character  (  -[-  ). 

Alteration  :  May  take  place  to  kaolin  or  muscovite,  by  loss  of  F  and  taking  up  of 
H.^0  and  alkalies. 

Distinguished  from : 

((?)   Quartz  by  cleavage  and  biaxial  character. 

((^)  Sillimanite  (when  topaz  is  in  radiating  aggregates)  by  lower  refraction  and 
double  refraction. 


72 


CHARACTERS  OF  MIXER ALS. 


Remarks  :  Common  in  greisen  and  all  granitic  rocks  containing  tin  ore.  When 
formed  by  "  fumarole  "  action  (tin  veins)  the  mineral  shows  rod-like  radiating  forms. 
It  is  insoluble  in  hydrochloric  acid.      H.,  8.      Sp.  gr.,  3.5. 

STAUROLITE. 

Anisotropic.  Biaxial.  Orthorhomhic. 

Composition:   Fe(A10)^(A10H)(SiOj2.   ''"t   varying,    may    contain   Mg   or  Mn 

i:=r  c  Elongation  ||  t'. 

Usual  Appearance  in  Sections :  Short,  flat  prisms,  which  may  be  twinned  at  90° 

or  60°,    Fig.  47,    or   grains.      Color  yellowish  to  reddish-brown.      Index  of  refraction 

rather  high  (w'=  1.741),  hence  j-e/ief 
marked  and  surface  rough.  Cleavage, 
both  prismatic  and  pinacoidal,  variable. 
Inclusions  of  minute  quartz  grains  and 
carljonaceous  matter  found  in  larger 
crystals,  but  not  in  microscopic  crystals. 
Fleochroism  distinct  but  not  strong, 
showing  red  1|  i  (direction  of  elonga- 
tion). Pleochroic  halos  may  surround 
inclusions. 

Crossed  Wicols :   Double  refraction 
weak    (;  —  a:=o.oio).      Interference 
colors  middle  first  order,  white  to  yel- 
low, etc.  (about  like  quartz).      Extinc- 
tion \n  general  parallel  or  symmetrical  (in  cross-sections)   to  cleavages  or  crystal  out- 
line.      In  convergent   light,  Ax.     pi.  ||   cc 
Poo  (100),    Fig.  48,   Bxg.  II  c,    axial  angle 
large  (2E'^  1 80- ) ;  optical  character  (-!-)• 
Alteration.      Rarely  takes  place. 
Distinguished  from :    Titanite,    see 
under  the  latter  mineral. 

Remarks:  Found  in  metamorphic 
schists,  associated  with  cyanite  (disthene), 
iolite  (cordierite),  andalusite,  etc.  It  is 
one  of  the  minerals  produced  by  thermal 
metamorphism,  hence    found  in    rocks    of 

granite  contact-zones.      It  does  not  occur   in  the   eruptive   rocks  or  in  schists  rich   in 
amphibole.      Staurolite  is  not  acted  on  by  hydrochloric  acid.      H.,  7  to  7.5.      Sp.  gr., 

THE  ORTHORHOMBIC  PYROXENES. 


Fig.  47.  —  Staurolite,  showing  twinning 
at  90°  b  and  60°  c  also  granular  quartz  in- 
clusions.     (  From  Reinisch.  ) 


Fig.  48.  —  Staurolite,  cross-section. 


Anisotropic. 


Enstatite  and  Hypersthene. 
Biaxial. 


Composition  :  (Mg.Fe)Si03 


^  =  c. 


Orthorhombic. 
Elongation  ||  c'. 


Enstatite  contains  little,  if  any,  Fe.  Hypersthene  contains  more 
Fe,  its  optical  characters  beginning  to  show  with  about  lOy^. 

Usual  Appearance  in  Sections  :  Irregularly  bounded  individuals 
(E)  or  rounded  prismatic-pyramidal   ciystals  (H).      Columnar  or 


THE  ORTHORHOMBIC  PYROXEXES. 


73 


fibrous  structure  ||  c  often  shows  in  (E).  Fig.  49.  Prism  angle 
about  92°.  Outline  of  cTrystal  sections  very  similar  to  that  of 
monoclinic  p}roxenes. 

Tzviuiiing.  —  Not  so  common  as  in  monoclinic  pyroxenes.  Par- 
allel growths  with  monoclinic  pyroxene  (diallage)  occur. 

Colo?-. — Varies  with  Fe  ^c,  (E)  colorless,  (H)  brownish. 

hidcx  of  Refraction.  — ;/'  =  1.665  ^o  1.723  (about  the  same  as  in 
monoclinic  p}-roxene),  hence  rc/iif  marked  and  surface  rough. 


Fig.  49. — Enstatite,  showing  columnar  or  fibrous  structure  ||  c  axis.  Xorite, 
Harzburg. 

Cleavage.  —  \"ariable,  parallel  to  prism  (angle  92°)  common  to 
all  pyroxenes.  Also  cleavage  or  parting  parallel  to  brachy  pina- 
coid  (cc  P  5c  ,  010)  (prominent)  and  macro  pinacoid  (x  P  co  ,  100). 

Inclusions.  —  Parallel  oriented,  brownish  plates  and  rods,  pro- 
ducing "  schiller  "  structure  oh  the  principal  cleavage  faces,  Fig. 
15.      Glass  inclusions  abundant  in  (H). 

Polarized  Light : 

Plcochroism.  —  Almost  absent  in  (E),  but  distinct  in  (H),  increas- 
ing with  Fe  /c.  The  change  in  color  ma}-  be  \-er)'  marked,  from 
brownish  red  to  greenish  ||  c. 

Crossed  Nicols : 

Double  Refraction.  — Weak,  much  weaker  than  in  the  monoclinic 
pyroxenes,  increasing  with  Fe  ^c.      (;-  —  «  =  0.0 10  to  0.013.) 

Interference    Colors.  —  Higher   first  order,   about   the   same 
quartz. 

Extinction.  —  Parallel  to  cleavages  in  longitudinal  sectio^ij^^Pc]*^ 


74  CHARACTERS  OF  MIXERALS. 

are  parallel  to  a  or  h,  and  bisectin<^  angles  of  intersecting  prismatic 
cleavages  in  basal  sections. 

Convergent  I^ight:  Axial  plane  parallel  to  brachy  pinacoid 
(co  P  CO  ,  oio),  z.  i\,  parallel  to  best  pinacoidal  cleavage.  Bx^^.  ||  r  (E), 
II  a  (H).  Axial  angles  large  (2E=gs°  to  >  i8o°).  Optical  char- 
acter for  (E)  (  +  ),  for  (H)  (  — ).  On  account  of  weak  double  refrac- 
tion the  interference  figures  are  not  very  marked. 

Alteration  :  Takes  place  to  bastite,  serpentine,  etc. 

Distinguished  from  :  The  Monoclinic  Pyroxenes  and  Amphi- 
BOLE.  —  See  under  these  species. 

Remarks  :  Found  in  the  granular  rocks  of  the  gabbro-peridotite  series,  also  in  the 
olivine  basalts  (E);  and  in  crystals  in  porphyritic  andesite  (H).  These  minerals  are  in 
general  not  attacked  by  acids.      H.,  5  to  6.      Sp.  gr.,  3.1  to  3.5. 

Bronzite  is  the  name  given  to  the  variety  containing  about  5  <-/(  Fe  and  having  the 
characteristic  bronzy  lustre  due  to  inclusions. 

Bastite  (an  alteration  product  of  the  orthorhombic  pyroxenes 
poor  in  Fe).  —  Composed  of  fibres,  often  traversed  by  irregular 
cracks.  Color  light  yellowish  or  greenish  and  index  of  refraction 
about  the  same  as  Canada  balsam.  Pleochroism  faint  (only  seen 
in  thick  sections),  the  greatest  absorption  taking  place  parallel  to 
the  fibres.  Double  refraction  weak  and  extinction  parallel  to  the 
fibres.  Axial  angle  large  and  axial  plane  at  right  angles  to  principal 
cleavage  face,  00  P  co  (oio).  The  position  of  the  axial  plane  is  the 
surest  distinction  between  bastite  and  the  orthorhombic  pyroxenes. 

CHRYSOLITE,  Olivine. 

Anisotropic.  Biaxial.  Orthorhombic. 

Composition  :  (Mg.Fe)2Si04.  Elongation  1|  a'  or  c'. 

Usual  Appearance  in  Sections  :  Prismatic  crystals  or  in  large 
angular  fragments  or  grains.  Longitudinal  sections  more  or  less 
lath-shaped,  with  pointed  ends,  Figs.  50  and  51,  cross-sections  eight 
sided.  Outlines  of  crystals  often  rounded  or  corroded.  Incipient 
forms  of  growth  may  occur,  and  sometimes  twinning  may  be 
observed. 

Color.  —  Nearly  colorless,  may  be  reddish  (with  high  Fe  /c). 

huiex  of  Refraction.  —  ;/=  1.679,  hence  relief  marked  and  sur 
face  rough. 

age.  —  Parallel   to  brachy  pinacoid  (co  ?  co  ,  qiq).  less  dis- 
iiT^^^mllel  to  macro  pinacoid  (co  P  co  ,  iqq),  Fig.  50.      Often  only 


CHRYSOLITE. 


7S 


made  visible  by  decomposition.  An  irregular  fracturing  occurs, 
which  increases  with  alteration  into  serpentine. 

Inclusions.  —  Chromite,  opaque  earths,  apatite  and  the  brown 
plates  so  common  in  hypersthene  ;  also  glass  and  slag  (in  basaltic 
rocks)  and  fluid  (in  peridotites  and  olivinfels). 

Polarized  Light : 

PleocroisDi.  —  In  general  none,  but  noticed  in  the  reddish  varie- 
ties, when  the  absorption  is  a  little  stronger  parallel  to  c. 

Crossed  Nicols : 

Double  Refraction.  — Very  strong  (;'  —  a  =  0.036). 

c=b 


\  I 

4     \. 


/ 


A-' 


\4 


Chrvsolite. 


Fig.  50. 
Basal  section. 


Fig.  51. 
Macro  pinacoid  section. 


hit  erf  ere)  ice  Colors.  —  Rather  high  in  order  (second  or  third), 
higher  than  the  colors  of  augite. 

Extinction.  —  In  general  parallel  to  cleavage  lines. 

Convergent  Light:  Axial  plane  parallel  to  base  (OP,  001)  and 
always  at  right  angles  to  cleavage  cracks,  Fig.  51.  Bx^.  \\  a. 
Axial  angle  very  large  (2y^>  180°).      Optical  character  (4-). 

Alteration  :  Into  serpentine  very  common,*  producing  "  mesh-" 
or  "  lattice  "-structure  (see  under  serpentine,  p.  109);  also  into 
amphibole,  etc.  In  certain  basaltic  rocks  the  rims  of  grains  may  be 
changed  into  goethite  ?,  and  in  certain  gabbros  the  crystals  may  be 
surrounded  by  a  radial  rim  of  amphibole. 

Distinguished  from  : 

Light-colored  Monoclinic  Pyroxenes.  —  By  absence  of  extinc- 
tion angles,  cleavage  (the  intersecting  prismatic  cleavages  of  augite 

*  For  other  alteration  processes,  see  Iddings^  Rosenlntsc/i,  p.  221.      1900. 


76  CHARACTERS  OF  M/XKRALS. 

being  of  equal  distinctness),  stronger  double  refraction  and  by 
axial  plane  being  parallel  to  base,  hence  always  parallel  or  at  right 
angles  to  cleavages  (in  augite  axial  plane  lies  in  clino  pinacoid,  bi- 
secting angles  of  intersecting  prismatic  cleavages).  Also  by  gela- 
tinization  with  acids. 

Remarks  :  Found  only  in  basic  rocks,  as  peridotite,  diabase,  gabbro,  norite,  basalts, 
etc.  Chrysolite  (olivine)  is  a  very  brittle  mineral  and  shows  under  mountain  making 
pressure  "  cataclastic "  structure.  Chromite  is  a  characteristic  associated  mineral. 
When  not  too  poor  in  Fe  chrysolite  becomes  permanently  red  and  pleochroic  when 
strongly  heated.  Chrysolite  is  decomposed  by  hydrochloric  and  sulphuric  acids,  with 
separation  of  gelatinous  silica.      H.,  6.5  to  7.      Sp.  gr.,  3.3  to  3.4. 

Hyalosidcrite  (a  more  ferruginous  chrysolite)  and  Fayalitc  (Fe,- 
SiOJ  are  reddish  in  sections,  and  common  in  the  basic  porphyritic 
eruptive  rocks. 

lOLITE,  Coraierite,  Dichroite. 
Anisotropic.  Biaxial.  Orthorhombic. 

Composition:  Mg,( Al.Fe)gSig028.  ('=  a.  Elongation  ||  a^. 

Usual  Appearance  in  Sections ;  Grains,  more  rarely  crystals  of  short  prismatic 
habit,  which  often  form  pseudo-hexagonal  interpenetration  twins.  Crystals  may  have 
edges  rounded  or  corroded.  Colorless,  but  may  be  bluish.  Index  of  refraction  a  little 
lower  than  quartz  {71'  =  1. 539),  hence  relief  low  and  surface  smooth.  Cleavage  very 
variable,  parallel  to  brachy  pinacoid  (00  Poo  ,  010),  especially  noticeable  when  decom- 
position has  taken  place.  Inclusions  of  sillimanite,  zircon,  rutile,  etc.,  may  be  seen. 
Pleochroism  usually  not  observed,  but  noticed  in  blue  sections  (yellowish  white  ||  <■  to 
blue).      Pleochroic  halos  (yellow)  surrounding  inclusions  common. 

Crossed  Nicols  :  Double  refraction  weak  (7 — (?  =  0.009),  ^i^e  quartz.  Interfer- 
ence colors  middle  first  order,  white  to  yellow.  Extinction  in  general  parallel  to 
cleavage  cracks.  In  convergent  light.  Ax.  pi.  ||oo  P(>o  ( loo),  Bx,i.  ||  c  ;  axial  angle  large 
(hyperbolas  only  seen  without  ellipses)  (2j5' =^  64°-i5o° )  ;  optical  character  (  ^  )•     * 

Alteration  :  Takes  place  readily,  forming  greenish  mica-like  decomposition 'prod- 
ucts, the  decomposition  commencing  along  the  crevices  or  about  the  inclusions. 

Distinguished  from  :  Quartz  by  observation  in  convergent  light  (quartz  is  uniax- 
ial), decomposition  and  pleochroism  or  pleochroic  halos.  The  section  can  also  be  treated 
with  hydrofluosilicic  acid,  when  the  evaporated  solution  yields  characteristic  prismatic 
crystals  of  magnesium  fluosilicate. 

Remarks:  Found  in  gneiss,  hornstone,  granite,  granulite,  etc.,  and  in  some  vol- 
canic rocks.  It  is  often  associated  with  garnet,  biotite,  sillimanite,  etc.  In  a  thick 
section  heating  to  redness  makes  the  pleochroism  more  distinct.  lolite  is  only  slightly 
acted  on  by  acids.  H.,  7  to  7.5.  Sp.  gr.,  2.6.  It  is  hard  to  make  a  mechanical  sep- 
aration from  quartz,  on  account  of  similarity  in  sp.  gr. 

NATROLITE. 

Anisotropic.  Biaxlvl.  Orthokhomhic. 

Composition:  XajAl(Al())(SiO.,)3 -f  aH^O.         r  =  c.         Elong.^tion  ||  c'. 

Usual  Appearance  in  Sections :  Aggregates  of  colorless,  fibrous  crystals,  which 
may  have  sphxTulitic  structure,  showing  a  dark  cross  between  crossed  nicols.      Index  of 


MONOCLINIC  PYROXENES.  y? 

refraction  lower  than  balsam  (w'^  1.483),  hence  (in  large  crystals)  the  surface  would 
appear  rather  rough. 

Crossed  Nicols  :  Double  refraction  weak  (;  — 0^0.012).  Interference  colors 
the  middle  first  order  (yellow,  etc.  ),  a  little  higher  than  those  of  quartz.  Extinction 
parallel  to  fibres.     Optical  character  (  +  )• 

Remarks  :  Never  a  primary  mineral  in  rocks,  but  found  in  igneous  rocks  filling 
amygdaloidal  cavities,  and  also  as  a  very  common  alteration  product  of  sodalite,  noselite, 
nephelite  and  acid  plagioclases.  It  gelatinizes  easily  with  hydrochloric  acid.  H.,  5  to 
5.5.      Sp.  gr.,  2.2. 

OTHER  ZEOLITES. 
Composition  :    Hydrous  silicates  ;   Al,Ca  and  Na  being  the  chief  bases. 
Usual   Appearance  in  Sections  :    The  form  depends  on  the  individual    mineral 
species,  but  the  majority  appear  in  elongated  crystals  or  fibres.      They  are  all  colorless 
and  most  of  them  have  a  small  index  of  refraction,  hence  no  ;-t'//ty"(prehnite  has  distinct 
relief). 

Crossed  Nicols:  The  double  refraction  is  generally  very  weak  (between  that  of 
nephelite  and  quartz),  giving  very  low  order  interference  colors  (prehnite  and  thom- 
sonite  have  strong  double  refraction). 

Rem.\rks  :  The  zeolites  are  always  secondary  minerals  in  rocks.  They  gelatinize 
with  hvdrochloric  acid. 

GYPSUM. 

AxisoTROi'ic.  Biaxial.  Monoclinic. 

CoMPOSlTlO.x  :  CaSO^  -^  211,0. 

Usual  Appearance  in  Sections  :  Colorless  grains  or  fibres.  May  be  colored,  how- 
ever, by  inclusic  ns  ot  carbonaceous  matter,  iron  oxides,  etc.  Index  of  refraction 
about  the  same  as  orthoclase  («'  =  1.525),  hence  no  relief  zxiA  surface  smooth.  Twin- 
ning lamella  abundant.  Cleavage  parallel  to  ooPob  (010)  gives  abundant  cracks,  other 
cleavages  may  also  be  noticed. 

Crossed  Nicols:  Double  refraction  weak  i^y  —  a  =^0.010),  the  same  as  quartz. 
Interference  colors  middle  first  order,  white  to  yellow.  Extinction  parallel  to  most 
perfect  cleavage  cracks  in  sections  parallel  to  b  axis ;  large  extinction  angles  noticed 
with  reference  to  less  perfect  cleavages.  In  convergent  light,  Ax.  pl.||  ocPob  (010),/.  e  || 
to  most  distinct  cleavages  ;  Bx„./\(- ^=  75°I5'' front ;  2i5'r=95°;  optical  character  (4-)- 
As  the  characters  of  gypsum  are  not  always  very  marked  it  may  be  necessary  to  employ 
micro-chemical  tests. 

Remarks  :  Forms  a  rock  by  itself,  often  associated  with  rock  salt.  It  also  occurs 
as  an  alternation  product  of  anhydrite.  Gypsum  is  soluble  in  hydrochloric  acid.  H., 
1.5  to  2.      Sp.  gr. ,  2.2  to  2.4. 

MONOCLINIC  PYROXENES,  Augite,  etc. 

Including  the  monoclinic  minerals  of  the  Pyroxene  Group,  which  show  distinctly  the 
characteristic  cleavage  parallel  to  an  almost  right-angled  prism. 

Anisotropic.  Biaxial.  Monoclinic. 

Elongation  ||  c'.* 
Composition  :  RSiO.^,  R  =  Ca,  Mg,  Mn,  Fe,  Al  chiefly,  with  the 
Ca  predominating  over  the  Mg. 

*  May  be  difficult  to  determine  in  the  case  of  prism  zone  sections,  showing  large  ex- 
tinction angles. 


78 


CHARACTERS  OF  MINERALS. 


Usual  Appearance  in  Sections  :  Both  in  crystals  and  more  or  less 
irregular  grains,  Figs.  4  and  i  2,  the  habit  varying  with  the  chem- 
ical composition  as  follows  : 

Diopside  (Ca,  Mg  varieties),  long  columnar  cr>'stals  and  grains. 

Augitc  (ditto,  but  containing  also  Al  and  Fej,  short  prismatic 
crystals  and  grains. 

Diallage,  granular  or  lamellar  (||  cc  P  :o  (lOO) ),  may  .show  fibrous 
structure  ||  c. 

Prism  angle  =  87°  6'  (important  in  cross-sections).  Sections  of 
cr>'stals  nearly  at  right  angles  to  the  vertical  axis  c  are  octagonal 
or  square  with  truncated  corners.  Figs.  4  and  33,  while  tho.se  par- 


FiG.  52.  — Augite,  section  parallel  to  c  axis  showing  prismatic  cleavage,  in  leucite- 
basalt.      (From  Cohen.) 

allel  to  the  c  axis  are  lath-shaped.  Pyroxene  also  occurs  in 
skeleton  crystals  and  acicular  microlites  in  eruptive  rocks. 

Zonal  structure  (especially  in  augites)  may  be  marked  b\'  dif- 
ferences in  color  or  extinction,  and  in  some  basalts  the  cr}-stals 
have  the  "  hour-glass  "  .structure. 

Tzvinnmg.  —  Common,  usually  the  twinning  plane  being  the 
orthopinacoid  (co  P  =c  ,  100).  Twin  lamclkv;  may  be  noticed. 
Intergrowths  occur  with  orthorhombic  pyroxene  and  amphibole. 

Color.  —  From  almost  colorless  through  green  (diop.sides,  Na 
pyroxenes,  etc.)  to  brown  (augites)  ;  the  red  to  brownish-red  color 
of  certain  augites  has  been  considered  due  to  manganese.  Yellow 
color  \'eiy  rare. 

Index  of  Refractio)i.  —  //  =  1.68  to  1.72,  hence  relief  high  and 
surface  rousfh. 


MOXOCLIXIC  PYROXEXES. 


79 


/ 

\ 

^v^\ 

&=b 

XV 

^^A 

X  X^ 

Ji^\  / 

I  X     ^' 

^V  ) 

\  \  /. 

^    4. 

\  A   2l 

\  /no 

\-- 

\/ 

Fig.  53.  —  Diallage,  cross- 
section. 


Cleavage.  —  More  or  less  perfect  parallel  to  prism  of  '^'j°  06', 
Cleavage  cracks  distinct  and  numerous,  but  not  generally  running 
uninterruptedly  through  crystal,  Figs.  12  and  52.  Cleavage  not  so 
perfect  as  that  of  amphibole. 

Parting. — Diallage  and  diopside  have  distinct  parting  parallel  to 
ortho  pinacoid   (=cPco,ioo),    Fig.    53. 
Some  crystals  may  show  parting  parallel 
to  base  (OP,  001). 

Inclusions. — Tabular  microscopic  in- 
terpositions, similar  to  those  in  bronzite, 
may  occur  in  diallage.  The  iron-ores, 
apatite,  etc.,  may  occur  in  augite. 

Polarized  Light : 

Pleochroisni. — Usually  not  noticed,  and 
in  general  only  appearing  as  different 
shades  of  the  same  color.     In  some  cases 

(diallage,  fassaite  and  Xa  rich  augite)  well  marked,  a  and  c  green 
to  yellowish  green  and  (1  brownish  to  reddish-brown ;  hence 
pleochroism  not  intense  in  sections  showing  extinction  angles. 
When  Ti  is  present,  violet  parallel  to  b. 

Crossed  Nicols : 

Double  Refraction. — Strong  (;'—«  =  0.022  to  0.029),  being 
stronger  in  the  pale  or  colorless  pyroxenes. 

Interference  Colors. — Second  order,  hence  always  bright  tints. 

Extinction. — Symmetrical  in  sections  (through  b  axis)  showing 
intersecting  cleavage  lines,  in  such  cases  bisecting  the  angles  of  the 
cleavage.  In  sections  showing  parallel  cleavage  lines,  only  paral- 
lel in  ortho  pinacoid  (^  P  x  ,  100)  sections,  in  all  other  sections  an 
extinction  angle  being  observed.  The  maximum  extinction  angle 
is  large,  lies  in  the  obtuse  angle,  varies  with  the  chemical  composi- 
tion from  36°  30'  to  54°,  and  is  only  obtained  when  the  section  of 
the  cr\^stal  is  parallel  to  the  clino  pinacoid  (00  P  ^  ,  010),  Fig,  54, 
varying  from  this  angle  to  0°,  when  the  section  is  parallel  to  the 
ortho  pinacoid  (00  P  00  ,  100).  In  Ti  and  Xa  pyroxenes  the  inclined 
dispersion  is  so  great  that  extinctions  are  not  sharp,  but  instead  a 
change  takes  place  in  the  interference  color  from  bluish  to  brownish. 

Convergent  hight:  Axial  plane  parallel  to  clino  pinacoid 
(oc  P  cc  ,  010).  Fig.  54.  A  cleavage  flake  parallel  to  ortho  pina- 
coid (oc  P  CO  ,  100)  shows  the  emergence  of  an  optic  axis  (ortho- 


8o 


CHARACTERS  OF  MIXER ALS. 


rhombic  pyroxene  would  show  enierL,aMice  of  a  bisectrix).      Bx^_. 

f  ,1- =  36°  to  54°  front.      Axial  anL,^les  large  {2E  =  70°  to  112°). 

Optical  character  (-f).      The  interference  figures  are   distinct  on 

account  of  the  strong  double  refraction. 

Alteration  :   May  take  place  to  chlorite,  serpentine  or  amphibole 

(uralitization  *),  depending  on  the  chemical   composition   and   the 

conditions  producing  the  change. 
Distinguished  from  : 

(^a)  Orthokhomiuc  Pvroxexes. — B\-  extinction  angle,  the  ortho- 
rhombic  pyroxenes  having  always  parallel 
or  s)'mmetrical  extinction  in  sections  paral- 
lel to  a,  b  or  c,  and  by  higher  order  inter- 
ference colors.  Also  from  hypersthene  by 
absence  of,  or  much  fainter,  pleochroism. 
Diallage  and  bronzite  might  be, confused  on 
account  of  pronounced  pinacoidal  parting, 
fibrous  structure  and  inclusions  ;  but  may 
be  distinguished  by  the  presence  or  absence 
of  extinction  angles  and  also  by  the  posi- 
tion of  the  optic  axes,  relative  to  the  best 
cleavage  plates. 

{b)  Amphibole. — See  under  amphibole. 
(c)   Epidote  and   Chrysolite  (Olivine).     When  light  colored 

and  granular,  by  examination  in  convergent  light.      The  plane  of 

the  optic  axes  is  parallel  to  the  clino  pinacoid  (cc  P  oc  ,  Qio),  hence  to 

the  longitudinal  axis  and  cleavage  cracks,  while  in  epidote  it  is  at 

right  angles  to  these  directions  and  in  crysolite  parallel  to  the  base. 

Also  yellow  color  is  common  in  epidote  but  rare  in  pyroxene. 

Remarks  :  Next  to  the  feldspars  pyroxene  is  the  most  common  constituent  of  the 
igneous  rocks.  Diopside  and  fassaite  (green)  are  found  in  contact  rocks;  also,  what 
appear  to  be  the  same  pyroxenes,  in  many  eruptive  rocks,  as  andesites,  monzonites,  etc. 
Malacolite  (light  green)  is  found  in  amphibolites  and  eclogites  (where  it  may  be  asso- 
ciated with  a  greenish  amphibole  (smaragdite) ).  Diallage  (bladed  and  twinned)  occurs 
in  gabbros  and  pyroxenites.  Common  augite  (brown)  is  found  in  the  remaining  basic 
eruptive  rocks.      In  the  schists  the  pyroxene  is  colorless. 

Finally  augite  occurs  as  a  secondary  product  resulting  from  the  "  magmatic  resorp- 
tion "  of  hornblende  and  biotite. 

Chemical  corrosion  and  mechanical  deformation  may  occur.  The  green  and  brown 
augites  when  heated  to  redness  on  platinum  foil  may  become  red  in  color.  In  general 
the  pyroxenes  are  not  attacked  by  acids.      H.,  5  to  6.     Sp.   gr.,  2,-1  to  3.5.     The  sp. 


Fig.  54.  —  Diopside,  clino 
pinacoid    section. 


*  See  p.  85. 


AMPHIBOLE. 


gr.  of  the  pyroxenes  is  considerably  higher  than  that  of  the  amphiboles  of  similar  com- 
position, hence  mechanical  separations  are  possible. 

Aoiiitf  {^-Egirine)  (Na  pyroxenes). — Occur  in  green  or  brown, 
elongated  prismatic  crystals,  often  not  very  transparent  and  with 
marked  pleochroism  (like  amphibole).  Zonal  coloring  is  common. 
When  zonally  intergrown  with  pyroxene  the  outer  zone  is  aegirine. 
The  elongation  is  ||  a'  (distinction  from  amphibole  whose  elonga- 
tion is  II  c').  The  index  of  refraction  is  higher  than  in  the  other 
pyroxenes  (;/'  =  1-792)  and  the  double  refraction  stronger  {j  —  a 
=  0.040).  The  cxti)iction  angle  lies  in  the  acute  angle  and  is 
small  (5°)  and  the  optical  character  (  —  ). 

The  term  ^girine-atigitc  may  be  used  to  describe  a  soda,  pleo- 
chroic  augite  with  a  large  extinction  angle  (Weinschenk). 

These  pyroxenes  are  only  found  in  the  eruptive  rocks  rich  in  alkalies,  as  eljeolite- 
syenite,  phonolite,  certain  trachytes,  etc.;  hence  are  associated  with  elseolite,  sodalite, 
leucite,  etc.  The  small,  second  generation,  crystals,  in  the  ground  mass  of  a  rock,  are 
always  the  richest  in  Xa  of  the  pyroxenes  in  that  rock. 


Anisotropic. 


MONOCLINIC. 


AMPHIBOLE,  Hornblende,  etc. 
Biaxial. 

Eloxg.ation  11  c'. 

Composition  :    R/SiO^)^.       R  =  Mg,   Ca,  Fe  chiefly,  also  may 
contain  Al,  Xa,  Mn.      The  ?^Ig  predominates  o\'er  the  Ca. 


Fig.   55. — Hornblende,    showing   twinning  between  crossed   nicols,  in  amphibole- 
biotile-granite.      (From  Cohen.) 

Usual  Appearance  in  Sections  :   Both  in  ciystals  and  more  or 
less  irregular  grains,  Figs.  55  and   56,  the  habit  varying  with  the 
chemical  composition  as  follows  : 
6 


82  CHARACTERS  OF  MINERALS. 

Trcniolitc  (Mg.jCa)  and  Act'uioliic  ((M^Fe).jCa  varieties),  in  long 
columnar  to  needle-like  individuals,  with  no  terminal  planes  or 
frayed  out  ends.      May  be  in  dense  aggregates. 

Pargasitc  in  well  developed  crystals. 

Coviinon  green  HoDihlende  (aluminous  varieties)  in  crystals, 
compact  grains  or  shreds. 

Basaltie  Hornbleiuie  (iron  rich,  aluminous  varieties)  in  prismatic 
crystals  of  varying  length,  which  may  often  show  "  magmatic 
resorption  "  (to  augite  and  magnetite)  around  outer  zone  or 
throughout  whole  crystal. 

Crystals  are  simple  in  form  of  prismatic  habit,  with  prism  angle 
124°    30'.       Cross-sections  "are   acutely   rhombic,   generally  with 


Fig.  56.  •  Hornblende,  section  parallel  to  c  axis,  showing  prismatic  cleavage,  in 
hornblende-diorite.      (From  Cohen.) 

acute  angles  truncated,  hence  six-sided  (pyroxene  being  eight- 
sided).  Longitudinal  sections  are  lath-shaped  and  fibrous  struc- 
ture may  be  noticed.  Skeleton  crystals  may  also  occur,  being 
very  fine  in  certain  pitchstones. 

Zonal  structure  and  parallel  growth  may  be  noticed  in  the 
amphiboles. 

Tivinning. — Frequent,  parallel  to  ortho  pinacoid  (m  P55  ,  100). 
Twins  dual,  less  often  multiple.  Fig.  55.  Intergrowths  with 
pyroxene  and  biotite  occur. 

Color. — From  colorless  (tremolite),  through  green  (actinolite, 
pargasite  and  hornblende)  to  brown  (basaltic  hornblende).  Yellow 
in  some  \arieties  and  bluish  in  the  soda  varieties. 

Ineiex  of  Refraction. — n'  =  1.62 1  to  1.641  (1.7 19,  in  the  basaltic 
hornblende),  hence  /-r//^/ distinct  and  surface  rough. 


AMPHIBOLE. 


83 


Fig.   57. — Hornblende,   cross-section. 


Cleavage. — Perfect,  parallel  to  prism  of  1.24°  30'.  Generally 
appears  in  thin  sections  as  sharp  cracks  crowded  close  together, 
Figs.  56  aud  57.      More  perfect  than  in  pyroxene. 

Some  of  the  long  prisms  (actinolite  and  tremolite)  may  show 
transverse  parting. 

Inclusions. — The  iron  ores,  apatite,  etc.,  may  be  found  in  horn- 
blende. 

Polarized  Light : 

PlcocJiroisui. — All  colored  amphiboles  show  pleochroism,  which 
in  general  is  stronger  the 
darker  the  color  of  the  variety 
(actinolite  and  pargasite  show 
but  little).  The  absorption 
is  very  marked  in  the  horn- 
blendes, being  greatest  in  the 
general  direction  of  the  cleav- 
age lines  in  longitudinal  sec- 
tions.     Marked  differences  in 

absorption  are  also   characteristic   of  the  mineral   species  biotite, 
tourmaline  and  allanite.      Pleochroic  halos  (brownish)  surrounding 
inclusions  may  be  noticed. 
Crossed  Nicols : 

Double  Refraction. — Quite  strong,  but  a  little  weaker  than  in 
pyroxene  (-^  —  a  =  0.019  to  0.027).  F^^"" 
ruginous  basaltic  hornblende  has  strong 
double  refraction  (j-  —  a  =  0.072). 

Interference  Colors.  —  Second  order, 
hence  bright  tints.  The  colors  of  basaltic 
hornblende  are  so  high  that  they  show  no 
bright  tints. 

Extinction.  — Always  symmetrical  in  sec- 
tions (through  b  axis)  showing  intersecting 
cleavage  lines,  in  such  cases  bisecting  the 
angles  of  the  cleavage.  In  sections  show- 
ing parallel  cleavage  lines,  only  parallel  in 
ortho  pinacoid  (co  P  00  ,  100)  sections,  in  all 
other  sections  an  extinction  angle  being  ob- 
served. The  maximum  extinction  angle  lies  in  the  acute  angle 
and  is  much  smaller  than  in  pyroxene,  varying  with  the  chemical 


Fig.  58. — Actinolite, 
clino  pinacoid  section. 


84  CHARACTERS  OF  MINERALS. 

composition  from  o°-20°.  In  hornblende,  actinolite  and  tremolite 
i2°-20°,  Fig.  58  ;  in  the  basaltic  hornblende  o°-io°.  The  maxi- 
mum extinction  angle  is  only  obtained  when  the  section  of  the 
crystal  is  parallel  to  the  clino  pinacoid  (^  P  oo  ,  Oio),  varying  from 
this  angle  to  0°,  when  the  section  is  parallel  to  the  ortho  pinacoid 
(cc  P  CO  ,  100). 

Convergent  JJght:  Axial  plane  parallel  to  clino  pinacoid 
(00  P  So  ,  010),  Fig.  58.  Bx,,.  /,  c  =  o°-20°  behind.  Axial  angles 
large  {2E  =  yy^,  etc.).     Optical  character  (— ). 

Alteration  :  May  take  place  to  chlorite,  talc,  serpentine,  asbes- 
tus,  etc.,  depending  on  the  chemical  composition.  Amphibole 
frays  out  and  becomes  fibrous  during  alteration,  and  may  also 
lose  color. 

Distinguished  from  : 

(a)  Pyroxene.  —  By  usually  much  stronger  pleochroism  in  the 
colored  varieties,  and  by  cleavage  and  extinction  angle.  In  pyrox- 
ene the  cleavage  (parallel  to  prism  of  87°  06')  is  less  perfect; 
and  the  extinction  angle  is  much  larger,  varying  from  36°  to  54°. 

(b)  BiOTiTE.  —  By  the  extinction  in  the  mica  being  always 
about  parallel  to  the  cleavage.  Both  have  strong  absorption,  but 
biotite  shows  very  slight  pleochroism  in  sections  parallel  to  the 
cleavage,  and  has  only  the  one  cleavage  parallel  to  the  base.  Also 
the  biotite  has  lower  index  of  refraction  and  generally  shows  uni- 
axial interference  figure. 

Colorless  tremolite  may  be  distinguished  from  muscovite  and 
talc  by  extinction  angles,  relief  and  lower  order  interference  colors. 

(c)  Tourmaline.  —  By  presence  of  cleavage,  and  by  the  fact 
that  absorption  is  most  marked  about  parallel  to  the  elongation 
(also  to  parallel  cleavage  lines),  while  in  tourmaline  the  absorption 
is  strongest  at  right  angles  to  the  elongation. 

{d)  The  Orthorhombic  Pyroxenes.  —  By  extinction  angles, 
the  latter  having  parallel  extinction  in  all  sections  parallel  to  a, 
b  and  c,  and  by  prismatic  cleavage  of  124°  30'.  Pleochroism  is 
strong  in  the  colored  variedes  of  both  species,  but  in  amphibole 
it  appears  more  generally  as  a  variation  of  the  same  color  ;  while 
in  hypersthene  a  change  in  color  is  often  noticed,  from  brownish- 
red  to  greenish  parallel  to  c  axis. 

{e)  SiLLiMANiTE  and  Cyanite.  — See  under  the  latter. 


AMPHIBOLE.  85 

Remarks  :  Amphibole  comes  next  to  pyroxene  in  importance  and  distribution  of 
the  dark  colored  ferruginous  rock  forming  minerals.  As  a  rule  it  occurs  in  rocks  with 
a  large  percentage  of  SiOj,  associated  with  quartz  and  orthoclase  ;  while  augite  gener- 
ally occurs  in  rocks  of  a  basic  nature,  associated  with  plagioclase  and  little  or  no  free 
SiOj.  Furthermore  amphibole  contains  hydroxyl  and  is  therefore  naturally  found  in 
the  deep  eruptive  rocks  ;  its  place  being  taken  by  augite  in  the  effusives.  By  application 
of  heat  hornblende  changes  to  augite,  while  hydrochemical  processes  bring  about  the 
opposite  result  "  uralitization." 

Tremolite  and  actinolite  are  found  in  contact  rocks  and  crystalline  schists,  also  as  a 
result  of  the  alteration  of  olivine  into  serpentine.  Pargasite  occurs  in  contact  rocks. 
Common  green  hornblende  is  found  in  the  plutonic  rocks  (Na  poor  and  SiOj  rich),  also 
in  contact  rocks  and  crystalline  schists  (amphibolites).  Brown  hornblende  replaces  the 
green  variety  in  the  basic  plutonic  rocks.  Basaltic  hornblende  is  found  in  many  effusive 
Tocks. 

The  hornblende  crystals  in  eruptive  rocks,  being  among  the  first  formed  constituents, 
have  often  suffered  subsequent  corrosion  by  the  magma,  giving  rise  to  the  dark  border 
already  mentioned.  The  brown  primary  hornblende  in  some  rocks  may  be  changed  by 
a  process  analagous  to  "  uralitization  "  into  a  green,  reed-like  hornblende.  Mechanical 
•deformations  are  found  in  massive  and  schistose  rocks.  Light  green  amphiboles,  with 
weak  pleochroism,  may  often  be  colored  intensely  reddish-brown  and  made  strongly 
pleochroic  by  heating  to  redness  on  platinum  foil.  In  general  the  amphiboles  are  not 
afifected  by  acids.      H.,  5  to  6.      Sp.  gr.,  2.9  to  3.3. 

GlaucopJiaiic,  Arfvcdsonitc,  etc.  (Na  rich  amphiboles).  —  Occur 
blue  to  bluish-green  in  color  with  pleochroism,  and  weaker  double 
refraction  than  the  other  amphiboles.  Extinction  angles  vary  from 
4°-6°  (glaucophane)  to  14°  (aifvedsonite).  They  are  found  in 
contact  rocks,  crystalline  schists,  eclogite,  etc. 

For  the  rarer  and  less  known  members  of  the  amphibole  group, 
resource  should  be  had  to  more  elaborate  works. 

Uralitc.  —  Pyroxene  altered  to  amphibole,  having  the  outward 
crystal  form  of  pyroxene'  and  the  physical  characters  and  cleavage 
of  amphibole.  The  change  usually  commences  on  the  surface 
and  the  uralite  does  not  form  a  single  compact  crystal,  but  con- 
sists of  numerous  slender  columns  parallel  to  one  another.  These 
little  columns  or  fibers  have  their  c  and  b  axes  parallel  to  the  posi- 
tions of  these  axes  in  the  parent  mineral.  The  color  is  green  and 
the  pleochroism  weak  to  strong. 

This  change  is  called  "  uralitization  "  and  results  from  hydro- 
chemical  processes.  When  the  alteration  is  not  complete,  por- 
tions of  the  original  pyroxene  may  be  left,  having  all  the  char- 
acteristic optical  properties  of  this  latter  mineral. 

A)itJiopJiyllitt\  the  orthorhombic  amphibole,  with  always  parallel 
-extinction,  is  sometimes  found  in  colorless  to  brownish,  blade-  to 
rod-like  aggregates  in  crystalline  schists  and  serpentine. 


86 


CHARACTERS  OF  MINERALS. 


MICA  GROUP. 

Anisotropic.  Biaxial.  Moxoclinic. 

May  appear  hexagonal  or  orthorhombic. 

Composition  :  Elongation  (  ||  cleavage)  ||  c'. 

Biotitc  (black  or  ferro-magnesium  mica)  =  (H.K).,(Mg.Fe)2Al2 
(SiOJ.,,  approx. 

Phlogopitc  =  a  magnesium  mica,  near  biotite,  but  containing 
little  Fe. 

Miiscovitc  (white  or  potash  mica)  =  H2(K.Na)Al3(SiOJ3,  with 
some  replacement  by  Mg  or  Fe. 

Usual  Appearance  in  Sections  :  Scales,  which  may  be  notched 
or  jagged,  with  lateral  sections  lath-shaped  ;  or  shreds,  Fig.  59  c. 
When  distincdy  crystallized  (magnesium  micas)  the  thin  hexagonal 
plates  have  plane  angles  of  120",  Fig.  59  a.  Phlogopite  crystals 
may  be  extended  in  direction  of  c  axis. 


Fig.  59.  —  Mica.  A,  Biotite,  showing  liexagonal  cross-section  and  zonal  markings. 
Minette,  Freiberg.  B,  Biotite,  showing  strong  absorption  parallel  to  cleavage  and  also 
zonal  marking  (/"=:  plane  of  vibration  of  polarizer).  Minette,  Cumberland.  C,  Mus- 
covite in  bent  shreds  in  gneiss. 

Zonal  structure  not  uncommon  in  the  magnesium  micas,  Fig. 
59  A,  which  may  also  have  dark  iron  ore  border  like  hornblende. 

Tz^'iiiiiiiig.  —  Common,  generall}'  parallel  to  base  ;  seen  in 
sections  showing  cleavage  by  variations  in  extinction,  in  basal 
sections  by  distorted  interference  figures. 

Micas  of  different  kinds  often  associated  together  in  parallel- 
position,  also  intergrown  with  hornblende,  pyroxene,  chlorite  and 
quartz. 


MICA  GROUP.  '^7 

Color.  —  Depends  on  chemical  composition.  Biotites,  brown, 
green  or  red  to  almost  opaque.  Phlogopites,  colorless  or  yel- 
lowish.     Muscovites  colorless. 

Index  of  Refraction. —  //' =  1.564  to  1.6 18,  hence  somewhat 
marked  relief  diwd  surface  varies  in  appearance  from  slightly  rough 
to  fairly  rough.  In  polarized  light  the  surface  appears  roughest 
when  the  cleavage  cracks  are  parallel  to  the  plane  of  the  polarizer. 

Biotite  has  more  marked  relief 

Cleavage. — Very  perfect,  parallel  to  base  (OP,  00 1),  Fig.  60. 


Fig.  60.  — Biotite,  showing  basal  cleavage,  in  biotite-granite.      (From  Cohen.) 

Basal  sections  show  no  cleavage,  but  all  other  sections  show  many 
sharp,  parallel  cleavage  cracks. 

For  precussion  and  pressure  figures,  see  reference  given  below.* 

Inclusions.  —  !May  be  arranged  parallel  to  lines  of  pressure  figure. 
Rutile  needles,  tourmaline,  apatite,  etc.,  common  in  magnesium 
mica.      Zircon  inclusions  often  surrounded  by  pleochroic  halos. 

Polarized  Light : 

Pleochroisni.  —  Varies  with  the  color,  being  \'ery  marked  in  the 
colored  varieties  (from  pale  yellow  to  chestnut-brown  or  black). 
The  strong  absorption,  about  parallel  to  the  cleavage  lines,  is  very 
characteristic  of  the  colored  micas.  Fig.  59  b.  Strong  absorption 
is  also  noticed  in  hornblende,  tourmaline  and  allanite.  Absorption 
may  even  be  noticed  around  inclusions  (pleochroic  halos)  in  color- 
less, non-pleochroic  micas.  Cleavage  plates  of  biotite  are  not 
pleochroic  unless  the  axial  angle  is  large. 

*  Idding'  s  Rosenbusch,  p.  274. 


88  CflARACTERS  OF  MIXER ALS. 

Crossed  Nico  's  : 

Double  Refraction.  — Very  strong  (j-  —  n.  =  0.03410  0.058). 

bitcrfcrcHcc  Colors.  —  High  order  (third).  May  be  very  bright 
in  thin  sections  of  the  colorless  micas,  but  may  at  times  be  so  high 
in  order  as  not  to  show  any  bright  tints. 

Extinction.  — About  parallel  to  cleavage  lines.  \'ery  small  ex- 
tinction angles  may  be  noticed  in  biotites.  Basal  sections  of  bio- 
tite  (the  approximately  hexagonal  mica)  usually  appear  isotropic. 

Convergent  Light:  Axial  plane*  and  Bx^^.  practically  at  right 
angles  to  basal  cleavage  ;  therefore  cleavage  plates  always  show 
well  defined  interference  figures,  generally  biaxial  in  character. 
The  axial  angles  vary  greatly,  being  usually  small  for  biotite  and 
phlogopite  (may  appear  uniaxial)  and  large  formuscovite  {2E=  55° 
to  90°).      Optical  character  for  all  micas  (— ). 

Alteration  :  Biotites  decompose  quite  easily,  lose  color  and  may 
become  completely  bleached,  which  appears  to  be  due  to  a  leach- 
ing out  of  the  iron.  May  also  alter  to  green  chlorite,  with  a  fray- 
ing out  of  the  mica  and  a  change  to  chloritic  structure. 

Phlogopites  may  alter  to  fibrous,  scaly  masses,  apparently 
chiefly  talc.  "  Sagenite  "  webs  of  rutile  may  accompany  the 
alteration. 

Muscovites  are  characterized  by  their  freshness,  and  do  not  seem 
to  suffer  from  weathering. 

Distinguished  from  : 

{a)  Hornblende. — Magnesium  mica  has  extinction  about  paral- 
lel to  the  cleavage,  while  hornblende  ma)'  have  extinction  angles 
of  from  0°  to  20°.  Both  have  strong  absorption,  but  biotite  shows 
very  slight  pleochroism  in  basal  sections,  which  also  give  approxi- 
mately uniaxial  interference  figures  in  conxergent  light. 

{b)  Tourmaline.— Magnesium  mica  shreds  show  absorption 
parallel  to  elongation,  while  in  tourmaline  the  absorption  is  at  right 
angles  to  elongation.  There  is  also  an  absence  of  cleavage  in  tour- 
maline. 

(r)  Chlorite. —  By  strong  double  refraction,  the  very  high  order 
colors,  however,  being  often  not  noticed.  Chlorite  also  shows 
aggregate  structure  and  is  almost  always  greenish  in  color. 


*  Depending  on  whether  the  axial  plane  is  parallel  or  at  right  angles  to  the  clino 
pinacoid  (oc  P5b  ,  oio)  (the  plane  of  symmetry),  we  have  micas  of  the  second  (biotite)  or 
first  order  (muscovite). 


CHLORITE  GROUP.  89 

{d')  Talc. —  White  mica  by  large  axial  angle  of  scales  in  con- 
vergent light  and  by  micro-chemical  tests.  The  distinction  may 
be  \-er}'  difficult. 

Remarks  :  Muscovite  is  a  rare  primary  mineral  in  eruptive  rocks,  except  in  two- 
mica  granite,  etc.  As  a  secondary  mineral  it  occurs  in  dense  scaly  aggregrates  or  as 
pseudormorphs  after  feldspar,  nephelite,  etc.  It  is  frequent  in  crystalline  schists  and 
is  probably  also  the  mica  in  amphibolite  and  eclogite.  Phlogopite  is  found  chiefly  in 
contact  metamorphic  limestone  ;  and  may  be  distinguished  from  muscovite  by  nearly 
uniaxial  character  and  less  sharp  cleavage.  Biotite  is  much  more  widely  distributed, 
occurring  especially  in  eruptive  rocks,  cr)-stalline  schists  and  contact  rocks. 

Chemical  corrosion  occurs  in  original  biotite  of  porphyritic  rocks,  producing  a 
"resorption  border"  of  augite  and  magnetite.  Mechanical  deformations,  producing 
bending  and  slipping  along  "gliding"  planes  (oblique  to  cleavage),  are  common  to  all 
varieties  of  mica  and  may  produce  change  to  chlorite.  The  muscovites,  together  with 
the  feldspars,  are  the  most  characteristic  minerals  of  dynamo  metamorphic  origin. 
Biotites  and  phlogopites  are  attacked  by  sulphuric  acid  at  high  temperatures.  Musco- 
vite is  but  slightly  attacked  by  acids.  H.,  2  to  3.  Sp.  gr.,  2.7  to  3.2.  The  specific 
gravity  separation  between  the  micas  is  difficult  on  account  of  the  scaly  nature  of  the 
minerals. 

Other  micas  occur,  some  being  alteration  products  of  those 
already  described.      Among  these  may  be  mentioned  : 

LitJiia  Mica. —  Both  light  and  dark  colored,  occuring  in  granitic 
rocks  and  often  onh'  distinguished  chemically  from  muscovite 
and  biotite. 

Dainoiu'itc  (^Scricitc)  (h}'drous  K  mica). —  A  secondaiy  product 
usually  in  colorless,  fine  scaly  aggregates  in  phillites,  sericite- 
schists,  etc. 

CHLORITE    GROUP. 

Embracing  the  members  of  the  Chlorite  Group,  commonly  occurring  in  rocks. 

Anisotropic.  Biaxial.  Monoclintc. 

The  minerals  of  this  group  usually  appear  uniaxial,  and  crystallize  in  part  with  hex- 
agonal symmetry. 

Composition  :  May  be  considered  as  isomorphous  mixtures  of 
HXMgFe)3SiP9  and  H^(MgFe),(AlFe),SiO,j  (Rosenbusch). 

Usual  Appearance  in  Sections  :  Minute,  scaly  aggregates,  which 
ma\-  incline  to  radial  grouping  ;  or  in  minute  grains  as  a  pigment 
(veridite)  in  other  minerals. 

Tivinnitig. — May  be  seen  as  in  mica. 

Colo?-. — Generally  green,  varying  from  greenish  \\hite  to  dark 
green,  rarely  colorless  or  red. 

Index  of  Refraction. — //'  =  1.576  to  1.588,  hence  no  marked 
relief  2ind  only  slightl}"  rough  surface. 

Cleavage. — Like  mica,  very  perfect ;  parallel  to  flat  face,  which 


go  CHARACTERS  OF  MIXER ALS. 

is  considered  to  be  the  basal   plane.      This   cleavage   may  not  be 
noticed,  especially  in  secondary  chlorite  in  rocks. 

Polarized  Light : 

Plcocliroisin.- — In  green  and  yellow  tints  (green  ||  cleavage),  being 
more  marked  in  dark  colored  varieties.  Basal  sections  are  non- 
pleochroic,  the  mineral  being  practically  uniaxial.  Pleochroic  halos 
may  be  seen. 

Crossed  Nicols  : 

Double  Refraction. — Generally  very  weak  (j-  —  a.  =  o.ooi  to 
o.oi  i). 

Interference  Colors. — Ver}^  low  first  order,  gray  or  white,  at  times 
scarcely  noticed.  Anomalous  colors,  however,  often  seen  (deep 
blue  or  brown). 

Extinction. — Plates  parallel  to  cleavage  generally  appear  isotropic 
or  only  show  faint  color.  In  other  sections  extinction  is  appar- 
ently parallel  to  the  cleavage  in  the  uniaxial  type,  but  extinction 
angles  may  be  noticed  when  type  is  biaxial.  Complete  extinc- 
tion may  not  be  noticed,  due  to  aggregate  structure. 

Convergent  I^ight :   Plates  parallel  to  cleavage  show,  at  times, 
an  indistinct  interference  cross,  which  may  open  into  two  hyper- 
bolas, indicating  biaxial  nature  of  crystallization.      Ax.  pi.  ||  co  P  co 
(oio) ;  Bx^^.  /^  c  =  o°  to  15°  ;  2i5' variable.      Optical  character  (±). 

Distinguished  from  :  Serpentine. — By  general  green  color  (ser- 
pentine, with  exception  of  Fe  rich  variety,  is  colorless),  pleochroism 
and  frequent  anomalous  interference  colors  ;  but  the  distinction 
between  these  two  minerals  may  be  very  difficult.  Chlorite  may 
resemble  decomposed  or  green  mica  (mica  has,  however,  strong 
double  refraction).  The  different  species  in  the  chlorite  group 
cannot  usualh'  be  distinguished  in  rocks. 

Remarks  :  The  chlorites  are  essentially  secondary  minerals,  derived  from  the  alumin- 
ous silicates,  biotite,  augite,  garnet,  feldspar,  etc.  They  are  found  abundantly  in  chlorite- 
schists,  contact  rocks,  etc.,  and  as  pigment  (veridite)  in  altered  eruptive  rocks.  May 
occur  as  a  primary  constituent  of  eruptive  rocks,  often  in  parallel  growth  with  biotite. 
Chlorides  are  acted  on  by  hot  hydrochloric  acid,  and  decomposed  easily  by  sulphuric 
acid.  H.,  2  to  3.  Sp.  gr.,  2.6  to  2.96.  A  thin  section  heated  to  redness  on  platinum 
foil  loses  water  and  becomes  opaque.  Ferruginous  varieties  are  turned  reddish-brown 
to  black  by  heating  (serpentine,  as  it  contains  less  iron,  may  give  negative  results  with 
this  test). 

Delessite. — Found  in  sph?erulites,  filling  ca\ities  in  amj-gdaloidal 
basic  rocks,  and  in  pseudsomorphs.  It  appears  to  be  much  altered 
to  other  minerals. 


E  PI  DOTE. 


91 


MONOCLINIC  (Pseudo- Hexagonal). 
Elongation  11  c'. 


TALC. 

Anisotropic. 

Composition  :  H2Mg.((Si03)^. 

Usual  Appearance  in  Sections  :  In  fine  scaly,  colorless  aggregates.  Sections,  cut- 
ting across  the  scales,  would  show  rod-like  forms.  Index  of  refraction  only  a  little 
higher  than  balsam  («/^  1.572),  hence  no  marked  relief  and  only  slightly  rough  surface. 
Cleavage  perfect  parallel  to  base,  like  mica. 

Crossed  Nicols  :  Double  refraction  very  strong  [y  —  0  =  0.050).  Interference 
colors  third  order,  like  muscovite.  Extinction  parallel  to  basal  cleavage  lines.  In  con- 
vergent  light,  Ax.  pi.  ||  00  Fob  (100),  Bx„.  ||  c  ;  axial  angle  small  {^2E  =  io°-20°)  ; 
optical  character  ( — ). 

Distinguished  from  :  Muscovite  (with  which  it  is  easily  confused)  by  micro-chem- 
ical tests,  proving  absence  of  alkalies  and  Al  ;  and  often  by  the  more  aggregate  struc- 
ture of  the  talc.      Also  by  smaller  axial  angle  of  scales  in  convergent  light. 

Remarks  :  Found  mainly  in  metamorphic  schists,  etc.,  always  as  a  secondary  pro- 
duct.     It  is  insoluble  in  hydrochloric  acid.      H.,  i  to  1.5.      Sp.  gr.,  2.6  to  2.8. 


EPIDOTE. 

Anlsotropic.  Bi.axial.  Monoclinic. 

ELONpATION    II    a'  or  c'. 

Composition  :  Ca.,AU(A10H)  (SiOJ3,  with  some  Fe  replacing  Al. 

Usual  Appearance  in  Sections  :  Columnar  to  thick  tabular  crys- 
tals, more  or  less  elongated  parallel  to  ortho  axis  b,  Fig.  61,  or 
in  granular  aggregates. 

Ti.vinning. — May  occur  but  rarely  noticed.  Irregular  interpene- 
trations  common  with  other  members  of  the  group ;  also  parallel 
growths. 

Color.  —  Colorless  to  yellowish  (Fe  poor)  or  yellow  to  green- 
ish to  yellow-brown  (Fe  rich). 


/ 

\ 

/ 

\ 

\ 

\ 

^           / 

101 

Fig.  61. 
Ortho  pinacoid  section. 


Epidote. 


Fig.  62. 
Clino  pinacoid  section. 


Index  of  Refraction.  —  n'  =  1.75  i,  hence  relief  high  and  surface 
rough. 


92  CHARACTJ'IRS  OF  MIXliRALS. 

Cleavage.  —  Parallel  to  base  (OP,  ooi),  Figs.  6i  and  62,  imper- 
fect parallel  to  ortho  pinacoid(cc  P  cc  ,  100).  Basal  cleavage  cracks 
not  very  numerous  and  appear  parallel  to  general  direction  of 
elongation. 

Polarized  Light : 

Pieoehroisiu. —  Varies  with  the  color,  being  faint  in  the  light  col- 
ored varieties,  but  strong  when  the  color  is  marked  (Fe  rich). 

Crossed  Nicols : 

Double  Refraetioti.  — Variable,  often  very  strong  (;'  —  a.  =  0.038 
to  0.056). 

Inteifcrence  Colors.  —  High  (third)  orders.  Intergrowths  with 
other  members  of  the  group  are  clearly  shown  by  the  "flecked" 
interference  colors.  < 

Exlinction.  —  Parallel  to  clea\'age  in  sections  parallel  to  b  axis. 
In  other  sections  extinction  angles  vary,  see  Fig.  62. 

Convergent  IJght:  Axial  plane  |i  co  Poo  (oio),  /.  e.  at  right 
angles  to  the  elongation  of  crystal  and  cleavage  cracks,  Fig.  61. 
Bx^^.  I  J"  =^  3°  behind.  Basal  cleavage  flakes  show  the  almost  X 
emergence  of  an  optic  axis.  Axial  angles  very  large  (i'£^>  180°). 
Optical  character  ( — ). 

Alteration  :   Does  not  take  place  readily. 

Distinguished  from  :  Light  colored  Monoclinic  Pyroxene.  — 
By  having  plane  of  optic  axes  at  right  angles  to  cleavage  cracks 
and  direction  of  elongation  ;  while  in  pyroxene  plane  of  optic  axes 
is  parallel  to  parallel  prismatic  cleavage  cracks  or  bisects  the  angle 
between  intersecting  cracks.  Furthermore  the  yellow  color  is  rare 
in  pyroxene. 

Remarks  :  Epidote  is  essentially  a  secondary  mineral,  resulting  from  the  alteration 
of  the  feldspars  and  the  ferro-magnesium  silicates.  It  is  found  in  crystalline  schists 
(especially  those  containing  hornblende),  gneiss,  gabbro,  diorite,  diabase,  lime-silicate 
hornstones,  contact  rocks,  etc.  Epidote  is  partially  decomposed  by  hydrochloric  acid. 
The  Fe  rich  epidote  can  be  changed  to  an  intense  color  by  "glowing"  in  the  air. 
H.,  6  to  7.     Sp.  gr.,  3.32  to  3.45. 

Piedntotititc  (containing  Mn). — Red  in  color.  Very  pleochroic, 
red  to  yellow.  Found  in  crystalline  schists,  the  porph}-rite  of 
Scotland,  the  famous  "  Porfido  rosso  antico  "  of  P^gypt  and  in  cer- 
tain Japanese  mica  schists. 


TITAXITE.  93 

ZOISITE. 

Essentially  orthorhombic  ?  members  of  Epidote  group.  * 

Composition  :  Like  epidote  but  without  any  Fe. 

Usual  Appearance  in  Sections  :  Similar  to  epidote  or  in  columnar  aggregates. 
Often  intergrown  with  epidote. 

Distinguished  from  epidote  by  general  absence  of  color  (colorless  to  yellowish)  and 
pleochroism  ;  by  slightly  lower  refractive  index  («■'  =  1.699  to  1.720)  and  by  much 
weaker  double  refraction  (7  —  a  =  0.005  ^"d  less).  The  interference  colors  are  very 
low  order,  gray  to  white,  but  anomalous  colors  are  often  seen  (yellow  or  prussian  blue). 
Extinction  is  in  general  parallel  to  pinacoidal  cleavage  cracks  (except  in  clinozoisite). 

The  plane  of  the  optic  axes  may  be  parallel  or  at  right  angles  to  cleavage  cracks,  and 
the  optical  character  is  (  +  )• 

Remarks  :  Generally  a  secondary  mineral.  Found  in  crystalline  schists,  amphib- 
olites,  contact  rocks,  eclogite,  etc.  and  in  "  saussurite."  Maybe  hard  to  distinguish 
from  vesuvianite  and  apatite. 

ALLANITE,  Orthite. 

^lonoclinic  member  of  Epidote  group. 

Composition  :   Like  epidote  but  containing  cerium. 

Usual  Appearance  in  Sections  :  Similar  to  epidote  in  form  ;  but  distinguished  by 
brown  color  (may  be  also  almost  colorless),  strong  pleochroism  and  absorption,  and 
medium  to  weak  double  refraction.  Lamellar  twinning  clearly  seen  on  account  of 
oblique  extinction.  When  included  in  hornblende  and  mica  it  is  surrounded  by  pleo- 
chroic  halos.      «' ^  1.78  about.      Usually  perfectly  fresh. 

Remarks  :  Found  as  an  accessory  mineral  in  SiO^  rich  eruptive  rocks  and  connected 
crystalline  schists,  and  (light-colored)  in  amphibolite  and  eclogite. 

TITANITE,  Sphene. 
Anisotropic.  Bl\xial.  Monoclixic. 

Composition  :  CaSiTiO..  Elongation  ||  a',  t 

Usual  Appearance  in  Sections  :  Wedge-shaped  cry.stals  (in  Na 
rich  rock.s,  more  prismaticalh-  developed)  ;  grains,  which  maj'  be 
elongated  ;  and  aggregates  of  small  rounded  particles,  which  appear 
nearly  opaque.      Sections  of  crystals  commonly  acute  rhombs,  Fig. 

63-    ' 

Tii'inniiig. — Occurs,  the  twinning  boundary  bisecting  the  acute 
angles  of  the  rhomb  (only  noticed  between  crossed  nicols),  Fig.  64. 

*  The  distinction  between  zoisite  a  and  ,3  (essentially  orthorhombic,  but  may  be  com- 
posite tricHnic  twins)  and  clinozoisite  (monoclinic  close  to  orthorhombic)  depends  on 
differences  in  position  of  plane  of  optic  axes  ;  axial  figures  shown  by  cleavage  plates  ; 
dispersion  ;  anomalous  interference  colors  ;  etc.  See  Weinschenk's  Die  Gesieinbilden- 
den  JMineralie7i,  p.  83,  1901. 

f  Test  not  easily  made  on  account  of  the  very  high  order  interference  colors,  result- 
ing from  the  strong  double  refraction. 


94 


CHARACTERS  OF  MINERALS. 


Color. — Reddish-brown  to  yellowish  to  colorless. 
Index   of  Rcf radio )i. — ;/'  =  1.920  to    1.963,   hence  relief  \&xy 
marked  and  surface  very  rough. 

C I  e  a  V  age. — Imperfect 
and  not  parallel  to  predom- 
inant form,  hence  only  ap- 
pears as  a  few  rough  cracks, 
which  are  not  parallel  to 
any  crystallographic  bound- 
ary, Fig.  63.  Cleavage 
rarely  observed  in  secon- 
dary grains. 

Polarized  Light : 
PI  e  0  cJirois  in .  —  Varies 
with  the  color,  being  more  distinct  in  colored  crystals,  yellowish 
(the  lighter  color)  ||  a'  and  reddish-brown    ||  c'.      Scarcely  noticed 
when  the  color  is  light. 
Crossed  Nicols : 

Double  Refraction. — Very  strong  (j-  —  «  =  0.090  to  o.  141). 
Interference  Colors. — Very  high  order,  like  those  of  calcite.     Due 
to   the  fact  that  the   refractive  indices  of  two  of  the   rays  are  very 
nearly  alike,  some  sections  may  show  very  low  order  colors. 


Fig.  63. 


Titanite,  showing  acute  rhoinljic  cross' 
section. 


Fig.  64.  — Titanite,  showing  twinning  in  nephcline-syenite.      (From  Cohen.) 

Extinction. — Extinction  angles  not  characteristic.      There   may 
be  no  complete  extinction  in  white  light,  owing  to  dispersion. 

Convergent  I^ight:   On  account  of  the  very  strong  characteris- 


FELDSPAR    GROUP.  95 

tic  dispersion  of  the  optic  axes  (o  >  6»),  the  axial  angle  varies  a  good 
deal  with  the  color  of  the  light  used,  2E^  =  54°^  2£^  =  33°.  By 
using  colored  glasses  this  variation  in  the  axial  angle  can  be  seen. 
The  axial  plane  lies  in  the  clino  pinacoid,  hence  bisects  the  obtuse 
angle  in  the  rhombic  cross-section,  Fig.  63.  Bx^.  /\r  =  51°  front. 
The  optical  character  is  (  +  ). 

Alteration  :   May  take  place. 

Distinguished  from  : 

(c?)  Staurolite. —  In  convergent  light  the  axial  plane  is  shown 
to  be  in  the  shorter  diagonal  of  the  rhombic  cross-section,  while  in 
staurolite  it  is  in  the  longer  diagonal. 

(d)  RuTiLE. —  By  biaxial  character. 

(c)  Calcite. —  The  light  colored  titanite  (sphene),  in  absence  of 
twinning,  by  higher  index  of  refraction. 

Titanite  may  easily  be  confused  with  some  of  the  rarer  minerals. 

Remarks  :  Titanite  is  always  an  accessory  mineral  and  is  found  distributed  in  all 
rocks,  except  SiOj  rich  eruptives  and  magnesia  silicate  rocks.  As  a  secondary  mineral 
it  forms  rims  around  other  titanium  minerals  or  pseudomorphs  after  them  and  also  the 
principal  part  of  leticoxene.  It  is  partly  soluble  in  hot  hydrochloric  acid  and  com- 
pletely decomposed  by  sulphuric  acid.  IL,  5  to  5.5.  Sp.  gr.,  T^.'i,  to  3.7.  In  a 
specific  gravity  separation,  it  falls  with  the  ferruginous  minerals  (on  account  of  its 
density),  and  from  these  can  generally  be  separated  by  electro-magnetic  methods. 

FELDSPAR    GROUP. 

Orthoclase,  Microcline  and  the  Plagioclases. 

ORTHOCLASE. 

Anisotropic.  Biaxial.  Monoclinic. 

Elongation  (||  cleavage)  ||  a'. 

Composition  :  KAlSi,0^,  with  some  replacement  by  Na. 

Usual  Appearance  in  Sections  :  In  crystals  and  grains.  In 
porphyritic  rocks  habit  of  crystals  more  or  less  tabular  parallel  to 
clino  pinacoid  (x  P  co  ,  010),  or  rectangular  much  extended  paral- 
lel to  clino  axis  a,  with  cross-sections  six-sided  or  rectangular  to 
long  lath  shape,  Figs.  65  and  66.  Crystals  may  be  changed  into 
rounded  or  looped  grains  by  chemical  corrosion,  Fig.  6.  Adularia 
crystals  more  prismatically  developed,  giving  rhombic  sections. 
Dimensions  of  crystals  vary  extremely  ;  microlites  occur,  at  times 
forming  sphxrulitic  structure.     The  very  fine  grained  ground-mass, 


96 


CHARACTERS  OF  MIX  URALS. 


"  micro-felsite  "  (not  resolved  by  the  microscope),  consists  largely 
of  feldspar. 

Intergrowths  with  microcline  and  plagioclase  common,  forming 
"  microperthite  "  when  lamelLx-  are  microscopic.     May  be  in  zonal 


Orthoclase  cleavage  plates. 
Fig.  65.  —  Basal.  Fig.  66.  —  Clino  pinacoid. 

formation  with  plagioclase  (the  orthoclase  on  the  periphery). 
Also  intergrown  with  quartz*  forming  "pegmatite"  and  "micro- 
pegmatite,"  Fig.  67. 

Zonal    structure  often   seen.  Fig.  20,  especially  when  decompo- 


FlG.  67.  —  Micro-pegmatitic    structure,    in    granophyric    quartz-porphyry.       (From 
Cohen. ) 

.sition  has  commenced  ;  and   in  fresh  crystals  may  be  indicated   by 
zonal  arrangement  of  inclusions. 

Tiviiiiiitig. — Very  common,  generalh'  after  Carlsbad\ayN,  Figs.  18 


*  See  under  quartz,  p.  60. 


FELDSPAR   GROUP. 


97 


and  69  ;  the  twinning  boundary,  dividing  the  section  longitudinally, 
either  being  parallel  to  edges  of  crystal  or  bent  or  jagged.  Twin- 
ning after  Bavcno  (twinning  boundary  diagonal,  \\\\\\  the  two  parts 
extinguishing  at  the  same  time,  but  having  a  and  c  directions, 
crossed  in  the  two  portions)  and  Ma)iebacJi  laws  less  common  * 

Color. — Colorless  or  tinged  by  oxide  of  iron.  Cloudy  if  decom- 
posed. 

Index  of  Refraction. — n'  =  1.523,  hence  no  relief  and  surface 
smooth. 

Cleavage. — Varies  and  sometimes  only  seen  in  very  thin  sec- 
tions, but  is  an  important  character  and  should  always  be  searched 
for.      It  occurs  perfect,  parallel  to  base  (OP,  001 ),  and  almost  as 


Fig.  68.  —  Orthoclase,  ortho  pinacoid  section  showing  cleavages  intersecting  at  90°, 
in  augite-syenite. 

perfect  parallel  to  clino  pinacoid  (co  P  ^  ,  010).     The  two  cleavages 
intersect  at  90°  in  sections  parallel  to  the  b  axis,  Fig.  68. 

Inclusions. — May  be  present  and  arranged  in   regular  or  zonal 
order,  but  not  important.      Do  not  occur  in  individuals  of  a  second 
generation. 
Polarized  Light : 

Pleochroisni. — None. 

Crossed  Nicols : 

Double  Refraction. — Very  weak  (-^  —  «  =  0.007). 

Interference  Colors. — Lower   first  order,  gray,    white,  etc.,   not 
quite  so  bright  as  colors  of  quartz  and  plagioclase. 

Extinction. — Being  monoclinic  the  extinction  angle  on  base  (OP. 

*  Idding' s  Rose7ibiisch,  p.  306. 
7 


9^  CflARACTERS  OF  MIXER ALS. 

ooi),  with  reference  to  clino  pinacoid  (co  P  cc  ,  Oio)  cleavage  cracks, 
is  o°.  On  clino  pinacoid,  with  reference  to  basal  cleavage  cracks, 
it  is  5°.  Some  sections  (notably  in  glassy  sanidine  grains)  may 
appear  dark  during  complete  rotation.  This  is  due  to  the  fact 
that  the  axial  angle  is  very  small  and  the  sections  act  approxi- 
mately like  those  of  a  uniaxial  mineral  at  right  angles  to  the  optic 
axis. 

Convergent  Itight:*  Plane  of  optic  axes  in  general  at  right 
angles  to  clino  pinacoid  (co  P  ^  ,  oio)  (plane  of  symmetry),  Fig.  65, 
hence  parallel  to  trace  of  basal  cleavage ;  but  in  some  sanidines 
parallel  to  plane  of  symmetry.  Bx^^.  /^  rt:  =  5°  above.  Axial 
angles  vary,  2E=  125°  (orthoclase),  o°-50°  (sanidine). t  May 
appear  uniaxial  when  axial  angle  is  very  small.  Optical  charac- 
ter  (  -  ). 

Alteration :  Very  common  to  clay;};,  muscovite,  hydrargillite, 
etc.  Generally  commences  along  the  cleavage  cracks,  and  when 
it  has  progressed  very  far  the  whole  feldspar  appears  opaque 
or  cloudy,  and  no  perceptible  change  takes  place  between  crossed 
nicols.  As  decomposition  is  very  prevalent  in  many  rocks,  the 
orthoclase  is  rarely  clear  or  pellucid.  "  Kaolinization  "  always  first 
attacks  the  plagioclase  and  later  the  orthoclase  in  a  rock.  Epi- 
dote  is  often  formed  when  accessory  solutions  are  present. 

Distinguished  from  : 

(rt)  The  other  Feldspars  and  Melilite. —  See  under  the  latter 
minerals. 

{B)  QuARjz.  —  Feldspar  is  biaxial  but  when  occurring  in  clear 
glassy  grains  (notably  sanidine),  which  appear  uniaxial  in  conver- 
gent light,  may  resemble  quartz.  When  tested  the  optical  char- 
acter is  (— ),  while  that  of  quartz  is  (  +  ). 

Remarks  :  The  members  of  the  feldspar  group  are  the  widest  distributed  of  the 
rock-forming  minerals  and  their  recognition  is  of  the  utmost  importance  on  account  of 

*  On  account  of  the  weak  double  refraction  the  interference  figures  are  not  very  sharp 
or  well  defined  in  thin  sections.  In  most  cases  only  the  black  hyperbolas  are  seen, 
without  any  colored  curves. 

f  By  heating  feldspar  crystals  the  axial  angle  decreases  to  0°  and  then  increases  in 
the  plane  of  symmetry  (at  right  angles  to  its  former  position).  On  cooling  the  axial 
angle  returns  to  its  former  position  if  the  temperature  has  not  exceeded  500°  C.  If 
the  temperature  has  been  6oo°-icxx)°  C.  for  some  time  the  axial  angle  will  not  re- 
turn to  its  former  position.  This  fact  may  give  some  clew  as  to  the  temperature  at 
which  the  feldspar  crystals  formed. 

X  This  change  to  kaolin  or  clay  in  granite  is  called  by  Dolomieu  "  La  inaladie  dii 
granif.^' 


MICRO  CLINE.  99 

their  bearing  on  the  systematic  classification  of  rocks.  Orthoclase  is  found  as  an  essen- 
tial constituent  in  the  more  acid  plutonic  and  older  volcanic  rocks,  as  granite,  syenite, 
trachyte,  porphyry,  and  also  in  gneiss,  crystalline  schists,  more  seldom  in  contact  rocks 
and  subordinate  in  clastic  rocks. 

Chemical  corrosion  (producing  rounded  or  looped  grains),  Fig.  6,  and  mechanical 
deformation  (producing  angular,  sharp-edged,  broken  grains,  bending  and  undulatory 
extinction*).  Fig.  7,  occur  in  orthoclase.  When  a  rock  containing  feldspar  crystals  is 
shattered,  the  orthoclase  breaks  parallel  to  basal  cleavage  and  plagioclase  parallel  to 
twinning  plane.  Orthoclase  is  practically  insoluble  in  acids.  H.,  6  to  6.5.  Sp.  gr., 
2.56. 

Sanidiiic. — This  clear,  glassy  variety  of  orthoclase,  occurs  in 
the  later  eruptive  rocks,  rhyolite,  trachyte,  obsidian,  etc.     Sandine 


Fig.  69.  —  Sanidine,  showing  Carlsbad  twin  and  cross-parting,  in  nepheline-phono- 
lite.      (From  Cohen.) 

often  has  a  parting  parallel  to  ortho  pinacoid  (co  P  cc,  lOo),  which 
may  be  noticed  in  sections  so  thick  that  the  cleavage  is  not  seen, 
Fig.  69,  In  general  it  shows  no  sign  of  decomposition,  and  has  a 
smaller  axial  angle  than  orthoclase.  Inclusions  of  glass  are  more 
abundant  than  in  orthoclase. 

MICROCLINE. 

Anisotropic.  Biaxial.  Triclinic 

Composition  :  KAlSi30j.. 

Usual  Appearance  in  Sections  :  As  a  rock  constituent  in  irreg- 
ular grains. 

In  general  characters  like  orthoclase  and  distinguislicd  from  it 
and  the  plagioclases  by  characteristic  ''gridiron  "  structure  between 
crossed  nicols,  resulting  from  the  polysynthetic  twinning  after  both 

*  See  p.  30. 


lOO  CHARACTERS  1)1-  MIXER  A  LS. 

Albitc  and  Pcriclinc  laws,  Fig.  19.  This  crossed  twinning  will 
show  in  all  sections  except  those  parallel  to  the  brachy  pinacoid 
(co  P  o;,  010).  The  lamellaj  are  generally  thinner  than  in  the 
plagioclases  and  more  "  spindle-shaped."  * 

Furthermore  the  rather  obscure  triclinic  crystallization  is  shown 
by  an  extinction  angle  of  +  15°  on  basal  cleavage  plates  with  ref- 
erence to  brachy  pinacoid  (00  P  cc,  010)  cleavage  lines  (distinction 
from  orthoclase,  which  has  0°  extinction  angle). 

Remarks  :  Found  with  orthoclase,  often  almost  replacing  it,  in  granite,  syenite, 
gneiss,  etc.,  and  is  one  of  the  last  minerals  to  form.  It  is  notably  resistant  to  decom- 
position. A  structure  like  the  microcline  twin  structure  may  be  produced  in  orthoclase 
by  dynamic  action,  f 

THE  PLAGIOCLASES. 

Albite,  Oligoclase,  Labradorite,  Anorthite. 

Ani.sotropic.  Biaxial,  Triclinic. 

Elongation  i\\ Albitc  tw^in    lamellae)  ||   a'   (except  in  anorthite 
when  it  may  be  ||  a'  or  c'). 
Composition  :  % 

Albite,  NaAlSigOg. 

Oligoclase,  ;/  (NaAlSigOg)  +  CaAUSi.,0,,  or  ;/  Ab  +  An,  n=2 
to  6. 

Labradorite,  NaAlSigO^  +  n  (CaAl^Sip,),  or  Ab  +  ;/  An, ;/  =  i, 
2  or  3. 

Anorthite,  CaAl.,Si,Og. 

Usual  Appearance  in  Sections  :  Much  the  same  as  orthoclase. 
Lath-shaped  §  forms  and  microlites  very  common,  especially  in 
the  acid  series. 

Twinning. —  Polysynthetic,  after  A/bitc  law,  almost  universal ; 
the  twinning  appearing  between  crossed  nicols  as  a  series  of  dark 
and  light  bands,  bounded  by  parallel  edges.  Figs.  70  and  71.      The 

*  Hatch's  Introduction  to  the  Study  of  Petrology,  p.  33. 

t  J.  W.  Judd,  Geol.  Mag.  [3],  Vol.  VI.,  p.  243,  1889. 

X  The  plagioclases  have  rather  a  complex  composition  :  but  may  be  regarded  as 
forming  a  series  from  the  composition  NaAlSi308(  Ab)  to  the  composition  CaAl.SijOg 
(An),  consisting  for  the  most  part  of  isomorphous  mixtures  of  these  types,  with  some 
replacement  by  KAlSi.jO^.  The  compositions  of  only  a  few  of  the  common  plagioclases 
are  given  above. 

\  The  lath-shaped  feldspars,  moulding  the  augite,  give  to  diabases  the  so-called 
"ophitic"  structure.  Fig.  12.  The  peculiarity  of  this  structure  is  that  the  feldspars 
crystallized  before  the  augite,  which  is  contrary  to  the  usual  order  of  formation. 


THE  PLAGIOCLASES. 


lOI 


twin  lamellae  are  parallel  to  brachy  pinacoid  (oo  P  oo,  Oio),  hence 
not  observed  in  sections  parallel  to  this  pinacoid.  The  lamellae 
may  appear  irregular  and  interrupted,  and  seem  to  be  broader  in 


Fig.  70.  —  Plagioclase,  showing  narrow  lamellre,  in  diabase.      (From  Cohen.) 

the  basic  than  in  the  acid  series.  When  this  twinning  fails,  how- 
ever, as  in  the  basic  plagioclases  in  certain  metamorphic  rocks,  the 
determination  becomes  very  difficult.  In  some  cases  polysynthetic 
twinning,  after  both  Albitc  and  Pericline  laws,  may  take  place  at 
the  same  time,  erivingf  rise  to  a  structure  somewhat  similar  to  that 


Fig.  71.  —  Plagioclase,  showing  Ijroad  lamellee,  in  gabbro.       (From  Cohen. 

of  microcline,  Fig.  72.      In  addition  the  polysynthetic  crystals  may 
be  twinned  like  orthoclase  after  Carlsbad  and  Baveno  laws. 

IIBRART 
yWIVERSITY  CF  CALIFOI^NIA 


R 


102  CHARACTERS  OF  MINERALS. 

The  general  characters    are   the    same  as    in  orthoclase  with  the 
following  differences  : 

Indices  of  Refraction  :;/'=  1.535  Albite. 

n'  =  1. 54 1  Oligoclase,  Ab^An,. 
n'  =  1.559  I-abradorite,  AbjAiij. 
n'  =  1.583  Anorthite. 
The  surface  of   anorthite  appears  sh'ghtly  rougher  than  that  of 
orthoclase. 

Cleavages,  parallel  to  base  (OP,  001)  and  brachy  pinacoid 
(co  P  CO,  010),  never  intersect  at  right  angles,  as  is  the  case  in  sec- 
tions of  orthoclase  parallel   to  b  axis.     This  is  due  to  the  triclinic 


Fig.  72.  —  Plagioclase,  showing  crossed  lamellae,  in  olivine-gabbro.       (From  Cohen.) 

system  of  crystallization,  but  the  divergence  from  a  right  angle  is 
small  (93°  36'  to  94°  10'). 

Inclusions  at  times  may  be  quite  important,  as  the  vitreous  in- 
clusions of  oligoclase  in  andesites,  etc.,  and  the  iron  ore  inclusions 
and  other  microlites  in  labradorite.  The  arrangement  of  these 
inclusions  may  be  zonal  or  in  parallel  orientation. 

Double  refraction  is  a  little  stronger  than  for  orthoclase  {y  —  o. 
=  0.008  to  0.013  (anorthite)),  hence  producing  slightly  brighter 
interference  colors  in  sections  of  the  same  thickness. 

Extinction  takes  place  in  all  sections  unsymmetrically  with 
respect  to  crystallographic,  twinning  or  cleavage  lines  (as  these 
minerals  are  triclinic)  ;  hence  extinction  angles  are  always  observed. 

Convergent  Light :  All  plagioclases  show  the  emergence  of  a 
bisectrix,"^'  more  or  less  oblique,  on  brachy  pinacoid  (00  P  oc,  010) 

*  For  the  positions  of  the  optic  axes,  bisectrices,  etc. ,  relative  to  the  cleavage  plates 
of  the  different  plagioclases,  see  Iddings'  Rosenbiisch,  p.  332. 


OPTICAL  DETERMINATION  OF  FLAG lOCLASES.        103 

cleavage  faces.  These  cleavage  faces  show  no  twin  lamellae,  un- 
less twinning  after  Pcricline  law  occurs,  in  which  case  the  determi- 
nation is  much  more  complicated.  The  axial  angle  is  large, 
2i5"=i55°  (albite).  Optical  character,  depending  on  variety, 
(  +  )  or  (-). 

Alteration :  Partly  the  same  as  in  orthoclase,  forming  clay, 
muscovite,  etc.  Calcite  and  epidote  are  more  common  as  side- 
products,  and  zeolitization  also  occurs  in  some  rocks.  The  plagio- 
clases  decompose  more  easily  than  orthoclase. 

Distinguished  from  : 

[a)  Orthoclase.  —  By  repeated  twinning  after  Alhitc  law,  giv- 
ing between  crossed  nicols  a  series  of  alternate  dark  and  light  bands. 

When  Albite  twinning  is  absent  the  distinction  is  very  difficult. 

{b)  MiCROCLiNE.  —  By  common  absence  of  the  microcline  ''grid- 
iron'' structure  between  crossed  nicols. 

Methods  for  Optical  Determination  of  the  Plagioclases. 

The  correct  determination  of  the  particular  plagioclase  is  of  the 
greatest  importance  in  the  classification  of  rocks,  and  it  is  no  longer 
sufficient  to  simply  determine  the  feldspar  as  either  orthoclase  or 
plagioclase. 

A  quantitative  analysis  of  isolated  material  would  lead  most 
surely  to  the  desired  result,  but  has  many  objections. 

Modern  optical  methods  now  permit  of  a  very  accurate  and  con- 
venient determination  under  ordinary  circumstances.  But  of 
course  these  methods  involve  a  knowledge  of  the  approximate 
orientation  of  the  section  tested.  When  this  section  is  not  a  defi- 
nite cleavage  fragment,  its  orientation  can  best  be  determined  by 
convergent  light  tests. 

Only  an  outline  of  these  methods  can  be  here  given,  and  refer- 
ence should  be  made  to  more  complete  works  *  for  an  elaborate 
discussion  of  the  subject. 

It  is  very  convenient  to  have  at  hand  a  set  of  glass  models  of 

*  Die  Gesteinsbildenden  Mineralien  (with  tables),  E.  Weinschenk,  Freiburg,  1901. 
Etude  sur  la  Determination  des  Feldspaths  dans  les  Plaques  Minces,  Michel  Levy, 
Paris,  1894  ;  and  The  Determination  of  the  Feldspars,  N.  H.  Winchell,  Am.  GeoL, 
Vol.  XXL,  No.  I,  1898.  hiding' s  Rosenhusch,  Wiley  &  Sons,  1900.  When  twin- 
ning is  present  after  both  Carlsbad  and  Albite  laws,  see  Etude  sur  la  Determination  des 
Feldspaths  (troisieme  fascicule),  Michel  Levy,  Paris,  1904. 


I04 


CHARACTERS  OF  MIX  URALS. 


the  plagioclases,  showing  location  of  plane  of  optic  axes,  vibra- 
tion directions  and  crystal  axes.* 

(i)  Schuster s  method  of  recognizing  the  different  feldspars  by 
extinction  angles  measured  on  the  cleavage  plates  t  is  very  pre- 
cise, but  not  always  applicable  for  crystals  in  rock  sections. 

Extinction  Angles  : 

on  base,  measured  from  trace  of  pina-  i  on   brachy    pinacoid,  measured    from 
coidal  cleavage.  trace  of  basal  cleavage. 


Albite 

Oligoclase,  Ab^An, 
Labradorite,  AbjAnj 
Anorthite 


-f         4°  j  Albite 

+         2°  j  Oligoclase,  Ab^  An^ 


+  19/2^ 
5  1/2°  i  Labradorite,  Ab,  An  I       —       20' 


—  36j^°  I  Anorthite 


Confusion  may  here  arise  between  albite  and  labradorite  if  dis- 
regard be  had  to  signs,  but  the  more  acid 
oligoclase  is  readily  distinguished  from  the 
basic  anorthite. 

By  convention  the  angles  on  base  and  pina- 
coids  are  (-|-)  when  the  direction  of  extinction 
has  apparently  moved  as  the  hands  of  a 
watch,  with  reference  to  the  upper  right  hand 
edge  (between  base  and  pinacoid)  of  the  crys- 
tal. When  the  reverse  is  true  the  angles  are 
(  -  ),  see  Fig.  Ji. 

(2)  The  method  of  Miclicl  Levy  and  others 
is  often  applicable,  especially  in  the  following 
case  : 

Sections  at  right  angles  to  the  brachy  pina- 
coid (x  P  ccT,  010)  and  hence  showing  Albite 
twinning.  —  These  sections,  as  nearly  perpen- 
dicular to  the  lamella;  as  possible,  are  known 
by  the  extinction  angles  on  each  side  of  the 
twin-line  being  approximately  the  same,  and  by  the  fact  that  equal 

*  Glass  models  of  the  feldspars  (size  20X10  cm.)  by  F.  Krantz,  Bonn.  Dia- 
grams, showing  optical  orientation  in  the  plagioclases,  in  Die  Gesteinsbildenden  Miner- 
alien,  E.  Weinschenk,  p.  133,  1901. 

t  For  this  method  of  investigation  little  cleavage  flakes  or  plates  can  often  be  obtained 
from  the  crushed  mineral,  but,  on  account  of  Albite  twinning,  plates  are  more  apt  to  be 
obtained  parallel  to  the  twinning  plane  than  to  the  best  basal  cleavage.  If  a  fragment 
with  only  one  cleavage  surface  is  obtained,  it  must  be  cemented  to  a  glass  by  this  sur- 
face and  ground  down  to  a  thin  section  with  parallel  sides. 


Fig.  73. —  Showing 
conventional  signs  of 
extinction  angles. 


OPTICAL  DETERMIXATIOX  OF  PLAGIOCLASES.  105 

"'  illumination "  of  the  two  adjacent  lamells;  is  obtained  when 
the  twin-line  is  parallel  to  the  plane  of  vibration  of  either  nicol. 
Measure  the  extinction  angles  in  as  many  sections  thus  selected  as 
possible  and  take  the  maximum  value.*  This  should  be  very 
close  to  the  maximum  extinction  angle,  which  is  a  constant  for 
each  kind  of  feldspar. 

Maximum  Extinction  Angles  in   Sections  Perpendicular  to  Albite  Twins 


Albite  16° 

Oligoclase,  Ab^Anj  5° 


Labradorite,  AbjAn^       27' 
Anorthite  5  3 ' 


In  the  determination  of  rod-like  microlites,  f  oligoclase  ex- 
tinguishes almost  parallel  to  its  length,  while  anorthite  may  show 
extinction  angles  of  over  27°.  When  these  microlites  show 
Albite  twinning  use  the  method  just  described. 

(3)  Fonqne's  method  %  can  be  used  when  the  optical  orientation 
of  the  section  is  known  (as  the  result  of  a  test  with  convergent 
light).  The  extinction  angles  of  these  known  sections  are  of  great 
diagnostic  importance. 

The  best  sections  are  those  at  right  angles  to  the  two  bisectri- 
ces, and  these  may  be  obtained  by  rapidly  testing  those  sections, 
in  the  rock,  which  show  an  interference  color  about  half  as  high  as 
the  maximum  color  in  the  rock  section,  in  this  way  avoiding  the 
sections  parallel  to  the  optic  axes. 

Having  found  such  a  section,  test  it  with  a  gypsum  plate  to 
prove  whether  the  bisectrix  is  X  a  or  c.  If  ±  a  ( these  sections 
show  sharp  twinning  striations)  measure  extinction  angle  between 

■*  This  test  is  only  possible  when  suitable  sections  of  the  given  feldspar  in  the  rock 
section  can  be  found.  The  method,  however,  can  be  used  with  great  accuracy  with 
the  aid  of  some  form  of  apparatus  for  properly  orienting  the  section.  See  "  Klein's 
Apparatus  for  the  Orientation  of  Thin  Sections,"  Sitzungsber.  Berlin.  Akad.,  1895, 
1151  ;  (also  in  N.  V.  Acad.  Sci.,  Vol.  XVI.,  p.  51,  1897)  ;  and  Von  Federov's  "Uni- 
versal Table,"  Zcil.fur  Kryst,  etc..  Vol.  XXV.,  p.  351. 

j  In  the  determination  of  feldspar  microlites  it  is  well  to  remember  the  following 
facts  :  "  Microcline  is  rarely,  or  never,  seen  in  the  condition  of  microlites,  while  the 
associations  of  labradorite  and  albite  are  so  different  that  there  is  little  danger  of  con- 
founding them.  Labradorite  is  the  commonest  product  of  the  consolidation  of  the  basic 
eruptives,  and  albite  almost  invariably  results  from  metamorphism,  frequently  from  the 
contact  of  igneous  rocks  on  the  calcareous  elastics."  N.  H.  Winchell,  Determination 
of  the  Feldspars,  Avi.  GeoL,  Vol.  XXI.,  No.  i,  p.  23^  1898. 

X  These  methods  (both  2  and  3)  are  often  not  applicable  on  account  of  the  tendency 
of  the  crystals  in  an  effusive  rock  to  parallel  orientation,  which  may  be  so  marked  that 
the  rock  section  does  not  show  any  favorable  sections  of  the  plagioclase. 


Io6  CHARACTERS  OF  MINERALS. 

trace  of  axial  plane  and  albitc  twinninij ;  if  X  c  measure  extinction 
an<^le  between  trace  of  axial  plane  and  basal  cleavage  cracks. 

Extinction  Angles  in  Sections:* 

1  C 


1  a 

Albite  74 

Oligoclase,  Ab^An,  88 
Labradorite,  Ab^An,  6o 
Anorthite  551^ 


5° 
22° 
48° 


When  both  extinction  angles  can  be  obtained,  the  determination 
of  the  plagioclase  is  very  certain,  but  the  result  cannot  be  re- 
garded as  definite  when  only  one  is  found  ;  and  the  method  be- 
comes more  difficult  as  the  crystals  become  smaller. 

The  position  of  the  axial  plane  should  be  determined  by  con- 
vergent light  test  and  not  simply  by  the  direction  of  extinction  in 
parallel  polarized  light. 

(4)  Bcckc' s  method  may  be  employed  to  identify  the  feldspar,  by 
determining  the  relative  values  of  the  indices  of  refraction  of  the 
feldspar  grain  when  it  lies  in  contact  with  a  quartz  grain  (best  re- 
sults) or  with  the  balsam  (not  such  good  results.)  The  grains 
should  have  vibration  directions  in  parallel  position. 

Table.! 
Orthoclase  ^  a  -v 

Microcline    ,-  [i  V  <  co  Quartz 

.  Albite  J  r)  ^ 

Oligoclase,  Ab^  An,  a      <  co  ;  y  >  co  Quartz 

Labradorite,  Ab,  An,    )      '  | 

A        .IV  r  /5  \  >  £  Quartz 

Anorthite  (        ( 

Other  methods  that  may  be  employed  are  here  simply  referred 
to  :   Michel   Levy's   method,  when   twinning  is  present  after  both 

*  These  extinction  angles,  as  well  as  those  previously  given,  are  those  of  only  a  few 
type  feldspars  of  definite  composition.  As  the  composition  varies  through  a  long  ser- 
ies, so  the  extinction  angle  changes,  one  being  a  function  of  the  other. 

For  a  complete  list  of  compositions  and  related  extinction  angles,  see  Iddings' 
Rosenbiisch  and  Levy  6^  Lacroix' s,  Les  Mitieiaux  des  Roc/ies. 

I  In  quartz  w  is  the  refractive  index  of  the  ray  with  vibration  direction  ||  a  [that  is 
the  direction  of  vibration  of  the  fastest  ray  (the  ordinary  ray)].  Hence  w  is  direction 
II  a  and  E  II  c.      In  the  feldspars  ;  «  ||  a,  >'  ||  C  and  /3  ||  5- 


OPTICAL  DETERMINATION  OF  PLAGIOCLASES.         lOJ 

Carlsbad  and  Albitc  laws  ;  determination  (in  convergent  light)  of 
the  emergence  of  an  optic  axis  with  reference  to  a  known  plane, 
the  basic  plagioclases  show  an  axis  about  parallel  to  c  of  the  crys- 
tal ;  determination  of  total  reflection  by  Wallerant's  total  reflecto- 
meter  ;  determinations  by  specific  gravity  separations,  by  use  of 
heavy  solutions,  and  chemical  and  micro-chemical  tests  (for  the 
relative  amounts  of  K,  Na  and  Ca.* 

Remarks:  The  plagioclases  may  have  the  same  two  general  habits  as  orthoclase, 
being  glassy  and  colorless  in  the  younger  eruptive  rocks,  and  dull  and  cloudy  in  the 
granular  and  porphyritic,  older,  massive  and  schistose  rocks.  They  occur  in  rocks  of 
intermediate  and  basic  composition. 

Albite  is  found  in  granite  (commonly  intergrown  with  orthoclase),  gneiss,  etc.,  and 
frequently  as  a  secondary  constituent  (secondary  feldspar  f)  in  the  feldspar  quartz 
mosaic  of  mechanically  metamorphosed  rocks.  It  may  also  be  present  in  acid  eruptive 
rocks. 

Oligoclase  is  very  frequent  in  granite,  syenite,  gneiss,  diorite,  trachyte,  andesite,  dia- 
base, etc. ;  and  particularly  accompanies  orthoclase. 

Labradorite  is  confined  more  to  the  gabbros,  J  basic  eruptive  rocks  and  crystalline 
schists,  rich  in  amphibole  and  pyroxene. 

Anorthite  occurs  in  gabbros,  the  most  basic  porphyrites,  basalts,  etc. 

Chemical  corrosion  and  mechanical  deformation,  §  may  take  place  as  in  orthoclase. 

Anorthite  and  labradorite  are  more  or  less  decomposed  by  hydrochloric  acid,  while 
albite  and  oligoclose  are  not  acted  on  by  the  acid. 

Especially  interesting  is  the  alteration  of  the  plagioclase  that  takes  place  in  gabbros, 
accompanied  by  "  uralitization  "  of  the  pyroxene,  forming  *'  saussurife.*'  This  consists 
of  a  white  to  greenish  confused  aggregate,  chiefly  of  zoisite,  grossularite,  vesuvianite, 
chlorite,  secondary  feldspar  (albite),  etc. 

Anortlioclasc  (a  Na  K,  triclinic  feldspar).  —  Shows  between 
crossed  nicols  intersecting  areas  of  exceedingly  fine  composite 
twin  structure  and  others  of  homogeneous  structure,  producing  a 
watery  or  "  moire  "  appearance.  The  twin  structure  may  be  only 
seen  in  very  thin  sections.  All  possible  kinds  of  perthitic  inter- 
growth  occur.  Further  distinguished  from  orthoclase  by  small 
extinction  angle  (4°)  on  base  and  by  smaller  axial  angle  {^2E 
=  72°  to  88°). 

*  These  tests  are  only  possible  on  pure  and  fresh  material.  The  specific  gravity  in- 
creases with  the  Ca  %   (albite  2.62,  anorthite  2.75). 

f  In  clear  unstriated  granules,  which  may  be  distinguished  from  quartz  by  biaxial 
interference  figure  in  convergent  light. 

J  The  tendency  of  labradorite  in  gabbros  to  twinning,  after  both  Albile  and  Pericline 
laws,  is  to  be  noted. 

^  Werveke's  (N.  J.  B.,  1883,  11,  97)  theory  is  that  a  twin  lamination  maybe 
caused  by  the  forces  producing  mechanical  deformations,  as  movement  in  the  magma 
and  mountain  making  pressure.  Such  lamellae  are  characterized  by  the  fact  that  their 
extent  and  course  seem  to  depend  on  fracture  lines  in  the  crystal. 


io8 


CHARACTERS  OF  MINERALS. 


Replaces  orthoclase  in  the  Na  rich  eruptives.  Found  in  augite-syenite  and  "  Rhom- 
henporphyr  "  of  Norway  (with  rhombic  cross-section),  acid  augite-andesite  of  Pantel- 
leria  and  in  the  porphyries  of  the  llartz. 


Anisotropic. 


CYANITE,  Disthene. 

Biaxial. 


Triclinic. 

Composition  :   (AlO),  SiO.,.  Elongation  1|  c'. 

Usual  Appearance  in  Sections  :  Blade-like  crystals  without  terminal  planes,  but 
with  cross-sections  (six  sided)  showing  two  long  parallel  edges  and  four  shorter  edges; 
also  in  columnar  aggregates.  Twinning  common,  with  generally  twinning  plane  paral- 
lel to  macro  pinacoid  (oo  Pec  ,  lOo).      Colorless  or  bluish  and  spotted.      The  index  of 

refraction  is  high  («'=  1.720),  hence  ;Y//£y  marked 
\    f  and    surface    rough.      Cleavage    perfect,    parallel    to 

^^-<-  macro  pinacoid    (00  Poo,    icx)),    appearing  as  sharp 

cracks,  parallel  to  longest  edges  in  cross-sections  ;  less 
distinct,  parallel  to  brachy  pinacoid  (00  Poo,  oio). 
Fibrous  parting  parallel  to  base  (OP,  001),  Fig.  74- 
Pleochroism  (colorless  to  blue  ||  c')  not  noticed  ex- 
cept in  colored  crystals. 

Crossed  Nicols  :  Double  refraction  quite  strong 
(;  — a  ^0.016).  Interference  colors  upper  first 
order,  yellow,  red,  violet,  etc.  Extmcticn  angles 
observed  in  all  sections  (being  triclinic),  reaching  a 
maximum  of  30^  on  macro  pinacoid  (00  P^,  lOo), 
Fig.  74.  Extinction  on  base,  about  parallel  to  most 
perfect  cleavage.  In  convergent  light  axial  angle 
large  ;  axial  plane  and  Bx„.  about  perpendicular  to 
best  cleavage  (00  Poo,  100);  optical  character  ( — ). 
Alteration  :  Seldom  observed,  but  may  take  place 
to  mica. 
Distinguished  from  : 

(rt)  A-MI'HIBOLE  by  cleavage  (intersecting  cleavages  at  124°  in  amphibole  and  90°  in 
cyanite)  and  by  00  P  60  (lOo)  cleavage  plates  of  cyanite  showing  emergence  of  acute 
bisectrix. 

{b)  Corundum  by  being  biaxial. 

Distinction  from  similar  appearing  minerals  may  be  difficult. 

Remarks  :  Found  in  gneiss,  granulite,  metamorphic  schists,  eclogite,  etc.,  commonly 
associated  with  garnet.      It  is  not  attacked  by  acids.      H.,  5  to  7.      Sp.  gr.,  3.6. 

SERPENTINE. 

Aggregate. 
Elongation  (of  fibres)  1|  c'. 
Composition  :  H^MggSi.^Og,  with  replacement  by  Fe. 
Usual  Appearance  in  Sections  :    Dense,  fibrous  (chrysotile)  or 
scaly  (antigorite)  aggregates. 

Color.  —  Colorless  to  light  greenish,  except  the  Fe  rich  variety 
which  is  green. 


Fig.    74.  —  Cyanite,   macro 
pinacoid  cleavage  section. 


SERPENTINE.  109 

Index  of  Refraction. — ;/' =  1.56  (about  the  same  as  balsam), 
hence  no  /r/zV/and  surface  smooth. 

Polarized  Light : 

PlcocJiroisvi.  —  Not  seen  or  very  feeble,  except  in  the  Fe  rich 
variety. 

Crossed  Nicols : 

Double  Refraction.  —  Rather  weak  (;-  —  «  =  0.009  ^^  0.0 1 1). 

Interference  Colors.  —  Middle  first  order,  gray,  white,  yellow, 
etc.  Anomalous  colors  do  not  appear.  The  aggregate  structure 
is  distinctly  seen  between  crossed  nicols.  Due  to  compensation 
aggregates  may  appear  isotropic. 

Distinguished  from  :  Chlorite.  —  By  more  usual  absence  of 
color,  pleochroism  and  anomalous  interference  colors  ;  but  this  dis- 
tinction may  be  very  difficult. 

Remarks  :  Serpentine  (both  antigorite  and  chrysotile)  is  essentially  a  secondary 
mineral,  resulting  in  most  cases  from  the  alteration  of  chrysolite  (olivine),  Fig.  22, 
more  rarely  of  pyroxene  or  amphibole.*  The  alteration  of  olivine  to  antigorit  leads  to 
the  characteristic  "lattice  structure"  and  alteration  to  chrysotile  to  "mesh  structure." 
In  the  case  of  the  "  mesh  "  formation  the  alteration  starts  from  the  surface  and  cracks, 
producing  fibres  of  chrysotile,  which  stand  at  right  angles  to  these  edges  and  cracks.  As 
serpentinization  proceeds  new  cracks  form,  due  to  increase  in  volume,  and  the  process  may 
continue  until  complete  pseudomorphism  takes  place.  When  this  subsequent  serpentini- 
zation of  the  meshes  takes  place  the  resulting  serpentine  may  appear  almost  isotropic  f 
and  is  certainly  different  from  the  chrysotile  of  the  first  formed  veins  (Weinschenk). 
Pieces  of  the  parent  mineral  are  often  present. 

Serpentine  is  found  in  ophiolites,  the  altered  basic  igneous  rocks,  pyroxenites,  peri- 
dotites,  etc.,  and  as  a  primary  mineral  in  the  Central  Alps  peridotite,  intergrown  with 
fresh  olivine  (Weinschenk).  It  may  also  form  a  rock  by  itself.  Serpentine  is  attacked 
quite  strongly  by  hydrochloric  acid,  still  more  so  by  sulphuric  acid.  Common  serpentine 
is  not  altered  by  heating  (distinction  from  chlorite),  but  the  Fe  rich  variety  becomes 
brown  and  opaque.      H.,  2.5  to  4.      Sp.  gr.,  2.5  to  2.7. 

CLAY,  Kaolin. 

Composition  :  H^Al.,Si20g  (kaolinite).  Aggregate. 

Usual  Appearance  in  Sections  :  Fine,  scaly,  colorless  aggregates,  which  appear 
opaque  (due  to  porous  structure).  The  scales  show  basal  cleavage.  Index  of  refrac- 
tion is  about  the  same  as  balsam  [//  =  1.55  ),  hence  no  relief.  The  double  refraction  is 
weak  (7 — a  =  0.008). 

Distinguished  from:  Colorless  Mica  and  Hydrargillite  [(Al(OH).,),  which  as 
an  alteration  product  of  the  feldspars  is  often  confused  with  clay]  by  weak  double  re- 
fraction. 

Remarks  :  Clay  results  from  the  alteration  of  the  feldspars  (especially  the  plagio- 
clases),  elseolite,  scapolite  and  other  silicates.  Kaolinite  is  insoluble  in  hydrochloric 
but  decomposed  by  sulphuric  acid.      H.,  2.5.      Sp.  gr.,  2.6. 

*  The  derivation  from  pyroxene  and  amphibole  appears  to  be  doubtful,  see  Wein- 
schenk's  Gesteinsbildenden  Mineralien,  1901,  p.  121. 
I  Marker's  Petrology  for  Students,  p.  63,  1895. 


CHAPTER  V. 

Methods  of  Preparing  Sections.* 

The  methods  described  will  be  those  used  in  making  rock  sec- 
tions. The  general  principles  involved  are  the  same  for  both  rock 
and  crystal  sections  ;  although  more  difficulties  are  encountered 
in  making  the  latter,  due  to  the  sections  being  required  in  some 


Fig.  75. 

definite  direction   or  of  some  precise  thickness,  or  on  account  of 
the  possibly  fragile  or  brittle  character  of  the  crystals. 

In  the  case  of  rock  sections  any  section,  taken  "  at  will  "  through 
the  rock,  will  generally  do. 

*  Sections  can  be  obtained  from  Voight  &  Hochgesang,  Gottingen  ;  C.  Marchand,  rue 
Censier,  i6ter,  Paris,  and  G.  D.  Julien,  932  Bloomfield  St.,  Hoboken,  N.  J. 

I  1 1 


I  12 


Ml'/niODS    OF    PRI'JWRLW}    SECTIONS. 


Cutting  and  Grinding  Machines. 

Many  different  kinds  are  used,  the  power  being  furnished  by 
steam,  electric  motor*  or  by  the  hand  or  foot.  A  convenient  type 
of  combined  cutting  and  grinding  machine  t  is  shown  in  Fig.  75. 

The  chip  or  fragment  of  rock  to  be  sliced  by  this  machine 
should  be  comparatively  small  in  size,  and  may  be  cut  by  either 
the  v^ertical  diamond-saw  D,  in   the  slicing  case   .S".  C,  or  h\  the 


Fig.  76. 

horizontal  emery-disc  D  in  the  tray  T.  The  fragment  is  held  in 
position  by  the  guides,  5.  G.  and  R.  G.,  as  shown  in  the  cut.  The 
moistened  emery  can  be  kept  in  the  tray  7"  and  fed  to  the  disc  by 
a  spoon  or  some  other  device.  Diamond  saws  are  only  needed 
in  the  case  of  hard  rocks. 

For  cutting  sections  of  crystals  in  definite  directions  a  form  of 
small  hand  apparatus  %  is  shown  in  Fig.  "jG. 

*  A  Crocker-Wheeler  motor  of  14^  to  ^  H.  P.  will  be  large  enough  for  an  ordinary 
laboratory  machine. 

I  Made  by  G.  D.  Julien,  932  Bloomfield  St.,  Hobokeii,  N.  J.  Price  :  Power  lathes 
(complete),  ^^150.00  to  ^300.00  ;   Foot-treadle  lathes,  ^100.00  to  $175.00. 

J  Made  by  Voigt  and  Hochgesang,  Gottingen.     Price,  $15.00. 


CUTTING   AND    GRINDING   MACHINES. 


113 


This  cut  shows  how  the  plate,  to  which  the  crystal  is  cemented, 
can  be  given  definite  rotation  in  two  directions.  The  simpler 
crystal  holder,  appearing  loose  on  the  stand,  may  be  used  when  a 
section  is  required  parallel  to  the  face  of  the  crystal  cemented  to 
the  holder. 

In  these  machines  the  cutting  is  done  with  rotating  saws  of 
sheet-tin,  usually  charged  with  either  diamond  dust  or  emery. 

In  the  case  of  a  very  small  crystal  a  section,  parallel  to  any  de- 
sired face,  can  be  obtained  by  cementing  this  face  to  a  suitable 
frame  or  holder  and  grinding  down  by  hand  on  a  glass  plate  with 
emery,  the  final  polishing  being  given  with  rouge.  When  the  sec- 
tion required  is  not  parallel  to  a  crystallographic  face  or  cleavage 
it  must  be  verified  geometrically  with  reference  to  other  faces.  If 
the  crystals  are  soluble  in  water,  some  other  liquid,  as  a  brom- 
naphthalin  or  benzine  must  be  used  in  grinding  ;  and  very  fragile 
crystals  are  rubbed  down  on  a  ground  glass  plate  without  emery 
or  rouge,  simply  using  bromnaphthalin  or  some  other  appropriate 
liquid. 

When  sections  of  a  crystal  are  desired  with  strictly  parallel  faces, 
the  form  of  grinding  apparatus  shown  in  Fig.  "]"]  can  be  used. 


This  apparatus  consists  of  a  cylinder,  held  within  a  suitable 
frame  supported  on  three  set-screw^s  «,,  a.^  and  a^^  of  hard  steel. 
The  lower  surface  of  the  frame  and  consequently  the  bottom  of  the 
cylinder  can  be  adjusted  by  the  wedge  k  and  the  set-screws  so  that 
it  is  exactly  parallel  to  the  grinding  surface  of  the  glass  plate. 
The  crystal  to  be  ground  down,  /,  is  cemented  to  the  bottom  of 


114  METIWDS   OF  PRFPARING   SECTIONS. 

the  cylinder,  and  the  whole  apparatus  rubbed  over  the  grinding 
surface  of  the  plate.  The  pressure  of  the  hand  on  the  upper  part 
of  the  cylinder  regulates  the  pressure  of  the  crystal  on  the  grind- 
ing surface.  In  this  way  a  surface  is  obtained  which  is  exactly 
parallel  to  the  surface  cemented  to  the  cylinder. 

Sections  of  definite  thickness  can  also  be  obtained  by  using  this 
apparatus  ;  for  the  adjustments  with  the  wedge  /'  may  be  so 
arranged  that,  when  the  stop  /  in  the  cylinder  has  reached  the  bot- 
tom of  the  slot  in  the  frame,  the  lower  surface  of  the  cylinder  will 
be  the  required  distance  from  the  grinding  surface  of  the  glass 
plate.  The  wedge  is  graduated  so  that  the  value  of  one  division 
on  the  sloping  top  is  known  in  ;/////.  of  vertical  distance. 

Saws. 

Saws,*  of  about  six  inches  in  diameter  made  from  ^L  inch  sheet- 
tin,  are  convenient  for  general  work,  and  may  be  charged  with 
diamond  dust  by  the  operator  as  follows  :  f  Crush  one  or  two 
carats  of  rough  crystals  of  diamond-bort  in  a  small  steel  mortar 
to  about  the  condition  of  fine  sand,  care  being  taken  not  to  reduce 
the  diamond  to  powder.  Transfer  the  dust  to  a  piece  of  flat  iron 
and,  after  collecting  it  in  a  heap,  moisten  with  a  drop  of  oil.  The 
cutting  edge  of  the  tin  disk  must  now  be  prepared  by  making  a 
series  of  incisions  (Jg  inch  deep)  on  the  outer  margin.  This  can 
be  done  by  striking  the  disk  with  a  sharp,  thin  knife  edge.  The 
larger  the  number  of  incisions  and  the  closer  they  are  together  the 
better.  Charging  the  saw  is  accomplished  by  gently  hammering 
the  edge  against  the  iron  plate,  upon  which  is  the  paste  of  dia- 
mond dust  and  oil.  The  disk  must  be  slowly  rotated  during  this 
charging  process,  which  should  be  continued  until  all  portions  of 
the  edge  have  been  gone  over  two  or  three  times. 

Instead  of  charging  with  diamond  dust  the  saws  may  be  used 
with  emery.  The  edges  of  the  saws  should  first  be  "  upset,"  as 
just  described,  and  then  while  rotating  charged  with  emer)^  The 
emery  in  the  state  of  mud  can  be  applied  with  the  thumb  and 
fingers,  or  the  saw  can  be  allowed  to  pass  through  a  tray  of  emery 

*  Saws  charged  with  diamond  dust  can  be  obtained  from  Elisha  T.  Jenks,  Middle- 
boro,  Mass.  Foreign  saws  ^"  and  \o''  diam.  can  be  brought  from  G.  D.  Julien, 
Hoboken,  N.  J. 

■\  School  of  Mines  Quarterly,  Vol.  XL,  p.  32. 


GRINDIXG   PLATES   OR   LAPS.  I  15 

mud.      The  finer  grades  of  emery  (Nos.   100  to  120)   should  be 
used. 

The  method  of  mounting  saws,  of  course,  depends  on  the  way 
the  saws  and  spindles  are  constructed.  It  is  well,  however,  to 
have  the  hole  in  the  center  of  the  saw-disk  sufficiently  large  to 
allow  of  adjustment  for  accurate  centering. 

Cutting. 

During  the  process  of  cutting  a  lubricant  must  be  used  on  the 
saw.  Petroleum  has  been  recommended  by  some,  but  water  seems 
to  be  the  most  used.  Holding  a  piece  of  soap  against  the  cut- 
ting edge  will  also  reduce  friction.  It  is  convenient  to  carry  the 
water  to  the  saw^  through  a  very  small  lead  pipe,  which  permits  of 
easy  adjustment,  so  that  the  water  can  be  delivered  at  just  the 
right  spot,  the  flow  of  water,  of  course,  being  regulated  by  a  stop- 
cock. 

Another  method  for  cutting  sections  is  used  by  the  U.  S.  Geo- 
logical Survey  at  Washington,  D.  C.  The  apparatus  consists  of 
an  endless  wire  of  soft  iron,  about  -^.^  inches  in  diameter,  revolving 
over  two  pulleys  or  wheels,  one  of  which  is  driven  by  steam- 
power.  The  wire  is  charged  by  dipping  the  fingers  in  water,  tak- 
ing up  a  little  fine  (120  grade)  emery  and  holding  the  fingers 
against  the  wire  above  the  section.  The  wire  revolves  so  as  to 
cut  downwards,  thus  carrying  with  it  the  emery  taken  from  the 
fingers. 

In  cutting  sections  the  thickness,  of  course,  depends  on  the 
character  of  the  rock  ;  the  more  porous  and  fragile  the  rock  the 
thicker  the  section  should  be  made,  an  ordinary  section  being 
about  as  thick  as  a  silver  quarter  of  a  dollar. 

Very  often  by  skill  and  practice  a  suitably  shaped  chip  or  frag- 
ment can  be  detached  from  the  specimen  with  a  hammer  or  with  a 
hammer  and  cleavage  chisels,  thus  avoiding  the  delay  and  trouble 
involved  in  the  use  of  the  section  cutter. 

Grinding  Plates  or  Laps. 

Copper  laps,  about  7-8  inches  in  diameter  and  I/2  inch  thick, 
seem  to  be  best  for  general  purposes  ;  although  laps  made  of  lead 
or  cast  iron  are  also  used.      A  lead  lap,  being  soft,  holds  the  emery 


Il6  METHODS   OF  PRE  PAR  I XG   SECTIONS. 

well  and  thus  hastens  the  grinding  away  of  the  section  ;  but  at  the 
same  time  its  own  smooth  surface  is  soon  destroyed.  A  cast  iron 
lap  is  much  harder  and  retains  its  smooth  surface  better,  but  does 
not  hold  the  emery  so  well  and  therefore  retards  the  process  of 
grinding.  Copper,  being  intermediate  in  hardness  between  lead 
and  iron,  seems  to  combine  the  advantages  of  both. 

It  is  always  good  practice  to  have  at  least  two  laps,  one  for  the 
preliminary  grinding  with  coarse  emery  and  the  other  for  the  final 
grinding  with  the  finest  grades  of  emery.  In  this  way  one  lap  is 
always  kept  with  a  smooth  surface,  making  it  possible  to  give  a 
uniform,  plane  finish  to  the  section. 

The  surface  of  the  laps  should  not  be  exactly  plane,  but  turned 
or  finished  so  as  to  be  a  little  higher  (^V  of  an  inch  for  diameter  of 
8  inches)  at  the  axis  than  at  the  periphery  ;  the  idea  being  to 
compensate  for  the  more  rapid  abrasive  action  towards  the  periph- 
ery. 

Cementing. 

For  grinding  down  to  a  proper  thinness  for  transparency  it  is 
necessary  to  cement  the  fragment  of  rock  to  a  holder,  which  con- 
sists of  a  small  piece  of  plate  glass  about  yi  inch  thick.  If  the 
piece  of  rock  has  been  cut  by  a  good  diamond  saw  the  surface 
will  be  comparatively  smooth  and  uniform,  and  may  only  need 
polishing  (described  under  grinding)  before  cementing.  When, 
however,  the  piece  of  rock  is  in  the  shape  of  a  chip  or  rough  frag- 
ment it  is  first  necessary  to  prepare  a  smooth  ground  surface  on 
one  side  (described  under  grinding)  before  it  can  be  cemented  to 
the  glass  holder. 

The  pieces  of  plate-glass  used  should  be  free  from  flaws,  bubbles 
or  anything  tending  to  destroy  uniformity  of  surface,  and  should 
be  a  little  larger  than  the  chips  and  slices  of  rock  for  which  they 
are  to  act  as  holders.  Hardened  Canada  balsam,  or,  better  yet,  a 
cement  made  of  a  mixture  of  shellac  and  Venice  turpentine,* 
should  be  used. 

*  "  Half  a  pound  or  less  of  ordinary  shellac  is  melted  in  a  flat  bottomed  open  vessel 
over  a  Bunsen  burner.  Then  an  equal  quantity  of  Venice  turpentine  is  carefully  added 
under  constant  stirring.  The  mass  should  be  allowed  to  boil  for  about  ten  minutes, 
during  which  the  stirring  is  continued.  Then  small  quantities  are  poured  in  separate 
heaps  on  an  iron  plate  or  other  cold  surface  and  rolled  into  stkks  about  seven  inches 
long  and  half  an  inch  thick." 


GRINDING.  117 

The  process  of  cementing  is  carried  on  as  follows  :  Heat  the 
glass  support  over  an  alcohol  lamp  or  a  Bunsen  burner,  place  it  on 
a  good  non-conductor  of  heat,  as  a  piece  of  wood,  and  rub  it  with 
a  stick  of  the  cement  until  a  sufficient  quantity  has  been  melted 
off.  The  piece  of  rock  should  then  be  treated  in  the  same  manner, 
applying  the  cement  to  the  smooth  finished  portion.  Care  must 
be  taken  that  neither  the  glass  nor  rock  preparation  are  heated  too 
much,  as  this  would  cause  the  cement  to  smoke  or  bubble.  After 
placing  the  rock  preparation  and  plate  together,  use  quite  a  little 
pressure  in  order  to  drive  out  all  superfluous  cement.  Examine 
now,  through  the  glass,  the  contact  surface  and  see  that  it  is  en- 
tirely free  from  bubbles,  etc. 

Bubbles  are  usually  caused  by  overheating  of  the  parts  before 
cementing.  If  only  very  few  bubbles  are  seen,  they  may  gener- 
ally be  removed  by  moving  the  rock  preparation  rapidly  back  and 
forth  over  the  glass  plate.  If  the  bubbles  are  numerous  or  cannot 
be  removed  in  the  manner  just  described,  the  rock  preparation 
must  be  separated  from  the  glass  plate  and  the  cementing  done 
over  again. 

Bubbles  must  not  be  allowed  to  remain,  as  the  portions  of  the 
rock  over  them,  not  having  any  cement  backing,  are  ground  away 
leaving  holes  in  the  section. 

Grinding. 

After  the  rock  preparation  has  been  cemented  to  its  glass  holder 
it  is  ready  for  the  grinding  process,  for  which  are  used  horizontal 
laps,  mounted  on  a  vertical  spindle  in  the  tray  T,  Fig.  75.  The 
copper  lap  for  the  first  grinding  is  put  in  place,  and,  after  it  has 
commenced  to  rotate,  is  charged  with  emery  and  water.  This 
can  conveniently  be  done  by  dropping  on  the  emery  and  water, 
either  w^ith  the  fingers  or  with  a  small  bunch  of  rags  at  the  end  of 
a  stick.  A  large  camel's  hair  brush  may  be  used  for  putting  on 
the  finest  emery.  The  emery  is  distributed  over  the  rotating  lap 
by  the  motion  of  the  rock  preparation  during  grinding.  The  kind 
of  emery  used  for  this  first  grinding  depends  somewhat  on  the 
character  of  the  rock  section.  If  the  section  is  composed  of  a  hard, 
compact,  fine-grained  rock,  the  first  emery  used  may  be  quite 
coarse  ;  the  coarser  the  emery  the  more  rapid  being  the  grinding 


Il8  METHODS    OF  J'RKPARIXG   SECTIONS. 

down.  For  rough,  quick  work,  Nos.  8o  to  lOO  emeiy  can  be 
used  until  the  section  becomes  quite  thin  ;  then,  still  using  the 
same  lap,  substitute  the  finest  corn  emery  (next  coarsest  to  flour 
emery)  and  continue  grinding  until  the  section  becomes  quite 
translucent.  The  lap  must  then  be  changed,  and  the  grinding 
finished  with  the  finest  emery  dust. 

Too  great  care  cannot  be  given  to  cleanliness  while  doing  this 
sort  of  work.  A  single  grain  of  coarse  emery,  getting  on  the  lap 
or  section  during  the  final  grinding,  will  often  spoil  the  whole 
work  by  making  a  bad  scratch.  In  grinding  down  the  section, 
great  care  must  be  taken  to  see  that  the  grinding  surface  is  kept 
parallel  to  the  surface  cemented  to  the  glass.  The  section  is  firmly 
pressed  against  the  lap  by  means  of  two  or  three  fingers,  depending 
on  its  size,  and  uniform  pressure  should  be  maintained  over  all 
parts. 

The  section  should  be  examined  frequently,  and  if  it  is  noticed 
that  one  part  is  thicker  than  another,  place  the  section  on  the  lap 
so  that  the  thicker  part  is  towards  the  periphery,  and  put  a  little 
more  pressure  upon  this  part.  Gutting  or  grooving  the  copper 
lap  is  prevented  by  moving  the  section  from  center  to  periphery 
and  back,  at  the  same  time  giving  to  it  a  slight  rotary  motion.  It 
is  best  to  have  the  lap  revolve  from  right  to  left  (in  the  opposite 
way  from  the  hands  of  a  watch),  as,  holding  the  section  on  the 
right  side  of  the  lap,  it  is  easier  to  keep  it  in  place  by  a  pulling 
motion  rather  than  by  pushing  it  against  the  motion  of  the  lap. 
In  the  final  stages  of  grinding  and  polishing  great  precaution  must 
be  exercised,  as  one  or  two  turns  too  many  of  the  lap  will  often  tear 
away  a  large  part  of  the  very  thin  section.  The  section  should  be 
frequently  examined,  by  transmitted  light,  with  a  small,  low-power 
microscope,  a  drop  of  water  or  oil  being  put  on  the  section  to 
render  it  more  transparent.  If  the  section  is  thick  and  fastened 
to  the  plate-glass  with  a  good  deal  of  cement,  it  will  be  found  con- 
venient, after  the  section  has  been  ground  quite  thin,  to  remove 
most  of  this  cement  from  around  its  edges.  If  this  is  not  done, 
it  may  delay  the  final  work  of  grinding  down.  A  good  rock  sec- 
tion should  be  made  so  thin  that  all  the  transparent  and  translucent 
component  minerals  can  be  examined  and  studied.  Well  made 
rock  sections  average  in  thickness  from  0.03  to  0.05  mm.      The 


MOUNTING.  119 

last  part  of  the  final  grinding  and  polishing  can  be  done  by  the 
hand  on  a  ground-glass  plate,  using  the  finest  emery  dust  or  rouge 
and  water. 

Flour  emery  may  also  be  used  entirely  for  grinding,  and  the 
finest  emery  dust  for  polishing.  The  finer  emery  makes  better 
work,  although  it  takes  more  time,  and  is  much  safer  for  porous  or 
brittle  sections,  as  the  danger  of  tearing  away  pieces  of  the  section 
is  avoided.  In  the  case  of  a  porous  or  decomposed  rock  it  is  well 
before  grinding  to  boil  the  section  in  Canada  balsam  or  other 
equivalent  material  so  as  to  fill  all  the  pores  and  interstices,  thus 
making  the  section  more  compact  and  less  liable  to  chip  away. 
Sometimes  it  may  even  be  best  before  grinding  to  mount  the  sec- 
tion on  its  permanent  glass  slide,  so  that  when  it  has  been  ground 
thin  enough  the  cover  can  be  put  on  without  running  any  risk  in 
transferring  the  section.  The  only  objection  is  that  the  glass  slide 
may  be  a  little  scratched  by  the  emery. 

Mounting. 

When  the  section  is  sufficiently  thin  and  has  a  smooth  uniform 
surface  it  is  ready  to  be  transferred  and  mounted  upon  its  perma- 
nent glass  slide.  The  glass  plate,  which  has  held  the  section  dur- 
ing the  grinding,  is  gently  heated,  and,  as  soon  as  the  cement  be- 
gins to  soften,  the.  section  is  very  gently  pushed  off  into  a  shallow 
cup  or  evaporating  dish  containing  turpentine  or  alcohol.  Turpen- 
tine is  convenient  for  general  use,  because  common  alcohol  (on 
account  of  the  water  it  contains)  unites  with  the  shellac  to  form  a 
kind  of  white  pasty  substance,  which  is  sometimes  hard  to  remove 
from  the  section.  The  section  should  be  carefully  cleaned  in  the 
turpentine  or  alcohol  bath  by  means  of  fine  camel's  hair  brushes. 
A  section-lifter,  made  from  a  piece  of  broad,  flat  watch-spring  fas- 
tened to  a  small  handle,  is  then  gently  placed  under  the  section, 
which  is  lifted  out  of  the  bath  in  an  incHned  position  so  as  to  allow 
the  liquid  to  drain  off  If  a  drop  still  adheres  it  can  be  removed 
by  touching  it  gently  with  the  finger. 

The  section  is  now  placed  in  a  tray,  upon  a  piece  of  white,  un- 
glazed  paper,  so  that  it  may  thoroughly  dry  ;  after  which  process 
it  is  transferred  to  a  glass  slide,  and,  if  necessary,  its  rough  edges 
chipped  off  with  a  knife. 


120  METHODS    OF  PRKPARIXG   SECTIONS. 

If  the  section  is  very  large,  and  it  is  desired  to  mount  it  in  two 
pieces,  it  can  easily  be  divided  by  holding  one  half  tightly  between 
two  glass  slides  and  gently  bending  the  other  half  with  the  fingers. 
The  fracture  will  take  place  along  the  edges  of  the  glass  slides. 

A  mixture  of  gum  damar  and  benzole  is  recommended  for 
mounting,  the  claim  being  made  that  gum  damar  does  not  turn 
yellow  with  age,  whereas  Canada  balsam  may  do  so.  Two  solutions 
of  gum  damar  and  benzole  are  used.  One  veiy  thin,  about  the 
consistency  of  water,  the  other  quite  thick,  about  the  consistency 
of  mucilage.  Both  solutions  are  prepared  by  dissolving  the  gum 
damar  in  benzole  and  filtering  through  a  linen  or  silk  rag.  If  the 
solution  is  too  thin  it  can  be  thickened  by  placing  the  bottle  in  a 
warm  place  and  allowing  the  excess  of  benzole  to  evaporate,  or  if 
too  thick,  more  benzole  can  be  added.  It  is  convenient  to  keep 
the  solutions  in  short  glass  bottles,  with  wide  necks  and  glass  cov- 
ers instead  of  corks.      The  solutions  can  be  applied  with  glass  rods. 

Wlien  the  section  is  all  ready  for  its  final  mounting  it  is  care- 
fulh"  centered  with  a  coarse  needle  on  the  glass  slide,*  and  a  drop 
of  the  thin  solution  placed  at  the  edge  of  the  section.  Capillary 
attraction  will  cause  it  to  flow  over  the  glass  slide  under  the  sec- 
tion, and  while  it  is  soft  the  section  can  be  again  adjusted  in  place. 
The  slide  is  then  placed  in  a  cool,  dust  proof  place  and  allowed  to 
stand  for  about  twelve  hours,  when  the  cement  will  be  set  and  the 
section  held  firmly  in  place.  The  upper  surface  of  the  section  is 
then  washed  with  a  drop  of  benzine,  and  as  much  as  may  be 
necessary  of  the  thick  mounting  solution  placed  upon  it  with  a 
glass  rod.  A  cover  of  glass  of  the  right  size  is  gently  heated  and 
placed  in  position  over  the  section.  Care  must  be  taken  not  to 
have  the  cover  too  hot,  as  it  would  cause  bubbles  to  form  in  the 
cement.  It  is  also  best  to  rest  one  edge  o"f  the  cover  on  the  cement 
first,  and  then  lower  it  gently  so  as  to  prevent  air  bubbles  from 
being  included.  The  cover  is  adjusted  and  then  held  firmly  in 
position  by  means  of  a  mounting  clamp,  which  presses  out  all  super- 
fluous cement.  If  a  few  small  bubbles  remain  near  the  edges  they 
may  be  let  alone,  as  they  will  generally  work  out  by  themselves  in 
time.     Sometimes  a  large  bubble  can  be  worked  out  by  local  pres- 

*  Square  glass  slides  are  recommended  instead  of  the  oblong  slides  (1x3  inches), 
which  are  very  apt  to  project  beyond  the  edges  of  the  stage  and  be  struck  by  the  fingers 
while  rotating  the  stage. 


CONVENIENT  APPARATUS  FOR    WORK. 


121 


sure  with  the  finger  and  gentle  heating  with  a  small  iron  rod.  If 
the  mounting  is  unsatisfactory  the  cover  should  be  removed  by 
heating,  the  old  cement  washed  off  with  a  camel' s-hair  brush  and 
benzole,  and  a  fresh  cover  put  on.  After  the  cement  has  set,  the 
clamp  may  be  removed,  but  the  slide  should  be  left  for  24  hours, 
or  longer  if  necessary,  until  the  cement  is  quite  hard.  This  takes 
longer  in  hot  than  in  cold  weather. 

The  advantage  of  using  two  solutions  for  mounting  is  that  the 
section  is  firmly  held  in  place  on  the  glass  slide  while  the  cover  is 
being  placed  over  it ;  there  is  thus  no  slipping  or  sliding  of  the 
section,  and  when  the  cover  is  adjusted  in  place  the  work  is  well 
finished  and  the  section  still  in  the  centre  of  the  slide. 

Canada  balsam,  or  balsam  in  xylol,  are  also  often  used  for 
mounting.  The  advantage  of  the  latter  being  its  quick  drying  or 
"  setting." 

Cleaning  and  Finishing. 

When  the  cement  has  set  the  superfluous  part  is  singed  by 
means  of  a  small  hot  iron  rod.  Care  must  be  taken  not  to  use  too 
large  a  rod  or  to  have  it  too  hot,  as  the  cement  under  the  cover 
might  then  soften  and  allow  the  cover  to  slip  or  air  bubbles  to 
form.  The  singeing  drives  off  the  more  volatile  part  of  the 
cement,  leaving  only  a  brittle  residue,  which  can  be  easily  scraped 
off  with  a  knife.  In  some  case  it  may  be  necessary  to  singe  and 
scrape  twice.  The  final  cleaning  is  done  with  a  soft  rag  and 
benzole.  The  slides  are  now  ready  to  be  marked,  either  with  a 
diamond  pencil  or  with  pasted  labels. 


Convenient  Apparatus  for  Work  in  a  Petrographical  Labora- 
tory: 


Section  lifters  made  of  watch  springs,  three 

sizes. 
Section  holders  or  clamps  for  pressing  out 

superfluous  cement. 
Needle  points  mounted  in  light  handles. 
Easy  spring  forceps. 
Mounting    frame    for  centering  section  on 

glass  slide. 
Small  iron  rod  for  singeing. 
Small  camel's  hair  brushes. 
Cutting  pliers  and  forceps. 
Set  of  wooden  section  trays. 


Glass  stopper  bottles  for  benzole,  benzine, 

turpentine  and  alcohol. 
Open  neck  bottles,  with  covers  for  the  two 

mounting  liquids. 
Rotating  mounting  stand. 
Small  microscope  for  testing  transparency 

of  section. 
Glass  rods. 

Small  squares  and  rectangles  of  plate-glass. 
Round,  oval  and  square  cover-glasses. 
Glass  slides  with  ground  edges. 


CHAPTER    VI. 
Chemical  and  Mechanical  Tests. 

These  tests  may  be  necessary  to  confirm  an  optical  determina- 
tion or  to  assist  in  the  differentiation  of  the  closely  related  species 
of  a  group,  as  for  example,  the  different  plagioclases  in  the  feldspar 
group.     They  may  also  be  useful  in  the  case  of  opaque  substances. 

The  ordinary  chemical  methods  employed  in  mineral  analyses 
are  often  not  applicable,  on  account  of  the  minute  size  of  the 
mineral  under  investigation  and  the  lack  of  sharpness  in  the  reac- 
tions. Those  methods*  are  to  be  preferred  which  produce  crystalli- 
zations independent  of  the  relative  proportions  of  the  materials  taking 
part  in  the  reaction  and  also  of  the  physical  conditions  involved. 

The  tests  can  be  made  either  on  the  crystal  in  a  rock  section  or 
on  the  isolated  crystal  or  fragment,  the  latter  method  being  pref- 
erable when  possible. 

Chemical  Tests  Made  on  Crystal  in  Section. 

The  part  of  the  section  to  be  tested  must  be  prepared  by  thor- 
ough washing  with  alcohol  and  benzole  to  remove  all  traces  of 
balsam.  If  the  section  is  covered,  the  cover-glass  can  be  cut 
across  with  a  diamond,  and,  after  heating,  the  desired  portion  re- 
moved with  a  knife  edge.  Any  portion  of  a  section  can  be  isolated 
by  surrounding  it  with  a  rim  of  viscous  balsam  or  by  putting  on  a 
new  cover-glass,  in  which  a  small  hole  has  been  made.f  This  hole 
can  be  accurately  adjusted  over  the  special  portion  of  the  section 
and  the  balsam  removed  by  alcohol  and  benzole. 

The  treatment  of  rock  sections  with  ordinary  acids,  such  as 
hydrochloric,  may  show  the  presence  of  easily  soluble  min- 
erals :|:  and  carbonates,  or  distinguish  silicates  that  are  soluble  with 
jelly,  or  produce  etched  figures  on  the  minerals. 

*  Reference  can  be  made  to  the  publications  on  this  subject  by  :  Borichy,  Behrens 
(Behren's  translation  by  Judd),  Haushofer,  Huysse,  Klement,  Streng,  Renard,  etc. 

t  In  case  an  acid  is  to  be  used  which  would  attack  glass,  the  cover-glass  can  be  re- 
placed by  a  thin  perforated  disk  of  platinum. 

X  The  bases  in  solution  can  be  determined  by  different  methods  of  analysis,  for  which 
the  student  is  referred  to  more  elaborate  works  on  this  subject.      In  some  cases  it  may 

123 


124  CHEMICAL    AND   MECHANICAL    TESTS. 

Test  for  Carbcviatcs.  Wlicn  only  present  in  very  minute  grains 
the  test  can  be  made  as  follows  :  Cover  the  rock  section  with  a 
drop  of  water  and  a  cover-glass,  then  allow  a  drop  of  acid  to 
slowly  diffuse  through  the  water  film.  The  glass  cover  will  pre- 
vent the  escape  of  the  gas  bubbles  *  which  will  thus  surely  be 
detected. 

If  necessary  the  section  can  be  warmed  by  heating  the  project- 
ing part  of  a  suitable  stand,  placed  under  the  section  on  the  stage 
of  the  microscope. 

lest  for  Gelatinizing  Silica.  Cover  the  carefully  cleansed  sec- 
tion with  a  little  dilute  acid  (commonly  hydrochloric)  and  let  stand. 
If  too  much  acid  is  used  the  resultant  gelatin  will  spread  over  the 
whole  section  and  not  appear  simply  on  the  gelatinizing  silicate. 
Warm  the  section,  if  necessary,  and  finally  rinse  off  the  remaining 
acid  thoroughly  with  water.  Do  not  allow  the  action  of  the  acid 
to  continue  too  long,  as  it  is  desirable  to  obtain  only  a  very  thin 
film  of  gelatinous  silica  over  the  minerals  attacked,  so  that  the 
optical  tests  can  still  be  made  on  these  minerals.  If  after  the  first 
trial  the  action  has  not  been  pronounced  enough,  the  test  should 
be  repeated. 

The  transparent  film  of  gelatinous  silica  is  made  more  visible 
by  covering  the  section  with  a  drop  of  water,  containing  a  dilute 
solution  of  fuchsine.  After  standing  for  some  time  the  section 
should  be  washed,  when  only  those  portions  covered  by  the  gela- 
tinous film  will  show  the  color  stain. 

Etched  Figures. f 

The  results  of  etching  tests  can  not  be  regarded  as  very  satis- 
factory in  the  case  of  sections  of  minerals  in  rock  sections,  on 
account  of  doubt  as  to  the  crystallographic  orientation  of  these 
sections. 

The  symmetry  of  the  etched  figures  depends  essentially  on  the 
relation  of  the  crystal  faces  on  which  they  are  obtained  to  the 
planes  of  symmetry. 

be  very  advantageous  to  treat  the  section  with  acid,  to  remove  certain  soluble  constitu- 
ents, when  other  minerals  not  distinctly  seen  at  first  may  be  made  more  apparent. 

*  The  gas  may  be  H.,S  from  a  soluble  sulphide,  in  which  case  the  solution  contain- 
ing the  bubbles  will  color  filter  paper  moistened  with  lead  water. 

t  Gcol.  and  Nat.  History  Stvuey  of  Minn.,  XIX.,  Ann.  Rept.,  p.  42  ;  also  A.  J. 
Moses,  Characters  of  Crystals,  p.  147. 


HEATING  SECTIONS    TO   REDNESS.  1 25 

The  forms  of  the  etched  figures  differ  on  the  same  face  of  a  min- 
eral, depending  on  the  reagent  used,  but  their  degree  of  symmetry 
is  independent  of  the  reagent  or  its  degree  of  concentration.  The 
sharpest  figures  are  produced  on  ciystal  and  cleavage  faces,  the 
figures  being  less  perfect  on  artificially  prepared  faces,  even  when 
polished.  The  etching  tests  may,  however,  be  tried  to  prove  the 
presence  of  twinning,  or  to  distinguish  between  minerals  of  similar 
appearance  but  belonging  to  different  systems.* 

Various  acids  or  alkalies  are  used  to  produce  the  etched  figures, 
depending  on  the  mineral  to  be  tested.  Different  factors  influence 
the  formation  of  good  etched  figures,  and  that  method  must  be  used 
which  seems  to  give  the  most  satisfactory  results  in  the  given  case. 
The  action  of  the  reagent  should  be  sufficiently  pronounced  to  de- 
velop clearly  the  etched  figures  ;  at  the  same  time  the  tests  must 
be  stopped  before  the  solvent  action  has  been  too  powerful. f  After 
treatment  for  etching  the  section  should  be  thoroughly  washed  and 
examined  in  some  fluid  of  weak  refraction  (with  ;/  lower  than  that 
of  the  crystal  section),  such  as  water  or  air.|  The  objective,  of 
course,  must  be  focused  on  the  surface  of  the  section. 

Heating  Sections  to  Redness.  § 

The  part  of  the  section  to  be  tested  must  be  removed  from  the 
object-glass,  carefully  cleansed  of  balsam,  and  held  on  platinum 
foil  in  the  oxidizing  blowpipe  flame.  After  the  test  the  fragment 
used  may  be  remounted  in  Canada  balsam  for  study. 

As  a  result  of  heating : 

Colorless,  hydrous  minerals  (zeolites  and  chlorites)  become 
cloudy  in  appearance. 

Colorless  silicates,  containing  protoxide  of  iron  (as  olivine,  or 
faintly  colored  pyroxene  or  amphibole),  become  red  or  reddish 
brown. 

*  The  symmetry  of  the  etched  figures  would,  of  course,  be  related  to  the  system  of 
crystallization. 

I  A.  J.  Moses,  Characters  of  Crystals,  Chap.  XVI. 

\  When  it  is  desired  to  preserve  the  section  and  at  the  same  time  to  study  the  sur- 
face covered  with  a  film  of  air,  the  edges  alone  of  the  cover-glass  should  be  cemented. 

§  Geol.  and  N'at.  Hist.  Swvey  of  Minn.,  XIX.,  Ann.  Rept.,  p.  50. 

Thin  sections  of  2-3  sq.  mm.  area  are  of  a  convenient  size,  and  they  should  be  sub- 
jected to  a  red  heat  for  1^-3  minutes.  Too  long  a  continuance  of  heat  may  render 
the  sections  too  dark  or  lessen  their  transparency  or  produce  melting. 


126  CHEMICAL   AND   MECHANICAL    TESTS. 

Colored  minerals  may  change  their  color,  chloritic  substances 
becoming  brown  or  black  if  heated  enough. 

Hornblende  always  becomes  pleochroic,  and  olivine  sometimes 
becomes  so. 

Members  of  the  sodalitc  group  may  be  turned  blue,  if  not  al- 
ready of  that  color. 

The  dichroism  (yellow  to  blue)  of  almost  colorless  iolite  may  be 
developed. 

Carbonaceous  particles  may  be  distinguished  from  the  iron  oxides 
by  being  consumed.* 

Methods  of  Isolating  Crystals  or  Mineral  Fragments  for 

Testing.! 

For  the  application  of  these  methods  the  rock  or  aggregate  of 
minerals  should  be  reduced  to  homogeneous  \  grains  of  uniform 
size  (preferably  crystals  or  cleavages)  and  not  to  powder.  This  is 
best  done  by  pounding  in  a  metal  mortar,  avoiding  all  grinding 
motion. 

The  separations  required  may  be  made  by  specific  gravity  solu- 
tions or  magnetic  methods,  alone  or  combined  ;  and  in  some  cases 
may  be  assisted  by  chemical  action. 

When  other  methods  fail  it  may  be  necessary  to  separate  single 
grains  from  a  mixture  by  hand.  A  grooved  piece  of  plate-glass, 
passed  beneath  the  objective  of  the  microscope,  will  be  found  use- 
ful. The  desired  grains  in  this  groove  can  be  picked  out  by  means 
of  a  piece  of  fine  waxed  thread  or  a  fine  pointed  stick  moistened  at 
the  end. 

The  hardness  of  the  homogeneous  grains  may  be  obtained  by 
pressing  them  firmly  into  the  end  of  a  lead  stamp  or  holder  and 
trying  the  effect  of  scratching  upon  the  faces  of  minerals  of  known 
hardness. 

*This  test  may  vary,  in  many  cases  graphite  not  being  consumed  even  after  long 
heating. 

■\  The  isolation  of  material  for  investigation  is  of  more  interest  for  the  lithologist  or 
chemist  than  for  the  student  of  optical  mineralogy  ;  therefore  only  a  very  brief  outline 
of  some  of  the  methods  employed  will  be  given. 

J  The  grains  passing  through  different  meshes  are  investigated  microscopically  to 
ascertain  which  size  grains  are  homogeneous  ;  the  rest  of  the  sample  should  then  be  re- 
duced to  grains  of  this  size. 


METHODS   OF  ISOLATING    CRYSTALS. 


127 


Specific  Gravity  Separation."^  Accomplished  by  use  of  fluids  of 
different  specific  gravity.  These  fluids  can  be  made  specifically 
lighter  by  dilution,  and  hence  the  fragments  will  fall  to  the  bottom 
in  order  of  decreasing  density.  Dilution  of  the  heavy  solutions  to 
any  specific  gravity  may  be  affected  empirically  until  the  solution 
will  just  suspend  a  fragment  of  a  mineral  having  the  desired  spe- 
cific gravity  ;  or  the  exact  specific  gravity  of  the  solution  may  be 
determined  by  the  Westphal  balance. 

The  following  indicators  may  be  employed  to  determine  the 
limits  of  the  specific  gravity  of  the  solution  to  be  used  for  separa- 
tion purposes  (V.  Goldschmidt) : 


No. 

Name. 

Locality. 

Sp.  Gr. 

No. 

Name. 

Locality. 

Sp   Gk 

I. 

Sulphur, 

Girgenti, 

2.070 

II. 

Quartz, 

Middleville, 

2.650 

2. 

Hyalite, 

Waltsch, 

2.160 

12. 

Labrador.ite, 

,  Labrador, 

2.689 

3- 

Opal, 

Scheiba, 

2.212 

13- 

Calcite, 

Rabenstein, 

2.715 

4- 

Natrolite, 

Brevig, 

2.246 

14. 

Dolomite, 

Muhrwinkel, 

2-733 

5- 

Pitchstone, 

Meissen, 

2.284 

15- 

Dolomite, 

Rauris, 

2.868 

6. 

Obsidian, 

Li  pari. 

2.362 

16. 

Prehnite, 

Kilpatrick, 

2.916 

7- 

Pearlite, 

Hungary, 

2.397 

17- 

Aragonite, 

Bilin, 

2-933 

8. 

Leucite, 

Vesuvius, 

2.465 

18. 

Actinolite, 

Zillerthal, 

3.020 

9- 

Adularia, 

St.  Gotthard, 

2.570 

19- 

Andalusite, 

Bodenmais, 

3-125 

10. 

Elasolite, 

Brevig, 

2.617 

20. 

Apatite, 

Ehrenfriedersdorf, 

3.180 

A  method  of  dilution,  to  any  desired  specific  gravity,  by  addi- 
tion of  a  measured  quantity  of  the  diluent  may  be  employed  by 
using  the  equation  t 

v{D  -  J) 


V,  = 


J-  I 


where  v  equals  volume  of  the  solution,  D  its  specific  gravity,  t\  the 
volume  of  the  diluent,!  and  J  the  density  desired. 

Among  the  heavy  solutions  employed  may  be  mentioned  : 

Thoulet's  solution  of  potassium-mercuric  iodide  (KI  :   Hglj  = 

I  :  1.24),   maximum  specific  gravity   3.196.      Klein's  solution  of 

cadmium  borotungstate  {2¥i.p,   2CdO,  BPg,  9W0O3  +  16H2O), 

maximum  specific  gravity  3.6.     These  two  solutions  can  be  mixed 

*  Further  reference  can  be  made  to  Idding' s  Rosenbusch,  p.  99.  A  convenient 
form  of  apparatus  is  also  described  in  School  of  Mines  Quarterly,  Vol.  X.,  p.  284,  1889. 

■\  Datia''s  Text  Book  of  Mineralogy,  p.  175,  14th  ed. 

+  The  diluent  is  usually  distilled  water.  In  some  cases  this  method  is  not  so  re- 
liable as  the  empirical  method,  especially  in  the  case  of  Thoulet's  solution  due  to  the 
contraction  which  takes  place. 


128  CHEMICAL  AND   MECHANICAL    TESTS. 

with  water  in  any  proportion  without  being  decomposed.  Solution 
of  barium  mercuric  iodide,  maximum  specific  gravity  3.588,  can- 
not be  diluted  with  water.  Methylene  iodide  (CH.,!^),  specific 
gravity  3.3243  at  i6°C.  varying  with  the  temperature,  can  be  di- 
luted with  benzole  but  not  with  water. 

Nitrates  of  silver  and  thallium*  (AgNOg :  TINO,  =  1:1)  fuse 
at  about  75 °C.  to  a  clear  mobile  liquid  with  specific  gravity  over 
4.5.  Can  be  mixed  while  melted  with  water  in  all  proportions; 
but  cannot  be  used  for  separation  of  sulphides,  as  they  are  attacked 
by  it. 

Possible  chemical  action  between  the  minerals  and  heavy  solu- 
tions must  not  be  overlooked  in  this  method  of  separation. 

The  funnel-shaped  apparatus  for  these  separations  must  be  so 
arranged,  with  stop-cocks,  etc.,  that  the  heavier  material  collected 
at  the  bottom  can  be  easily  drawn  off  or  removed  at  any  .stage  of 
dilution. 

These  separations,  for  various  reasons,  are  not  always  complete, 
but  the  best  results  are  obtained  when  the  processes  are  repeated 
several  times. 

Electro-magnetic  Separation.  All  iron-bearing  minerals  may  be 
separated  from  those  free  from  iron  by  an  electro-magnet. 

The  factors  influencing  the  attraction  of  a  mineral  by  an  electro- 
magnet are  not  definitely  known,  and  do  not  seem  to  depend  only 
on  the  percentage  of  iron. 

Minerals,  such  as  amphibole,  pyroxene,  epidote,  oli\ine  and 
garnet  (containing  iron),  may  often  be  separated  by  an  electro- 
magnet by  regulating  its  magnetic  intensity.! 

Separation  by  Chemical  Means.  Ver}-  man}-  different  methods 
may  be  used,  depending  on  the  nature  of  the  work  to  be  accom- 
plished ;  but  they  are  generally  only  reliable  in  the  hands  of  a 
good  chemist.  The  material  should  be  in  the  state  of  fine 
powder. 

As  an  example  may  be  mentioned  the  treatment  with  pure 
concentrated  HFl,  by  which  the  minerals  of  a  rock  are  attacked 
in  a  certain  sequence,  the  feldspars  and  related  minerals  first,  then 

*S.  L.  Penfield,  Am.  Jour.  Sci.,  Vol.  L.,  p.  446,  1895.  In  this  article  a  conve- 
nient form  of  separating  apparatus  is  also  described. 

f  A  convenient  list  of  minerals,  arranged  in  the  order  in  which  they  would  be  at- 
tracted by  increasing  the  force  of  the  electro-magnet,  is  given  in  Weinschenk's  Tabellen. 


BORICHY'S  METHOD.  I  29 

the  quartz  and  finally  the  ferro-magnesium  silicates,  such  as  am- 
phibole,  pyroxene,  olivine,  etc. 

Micro-Chemical  Reactions. 

The  first  requisite  is  to  bring  the  substance  to  be  investigated 
into  solution.  This  can  be  done  in  the  case  of  non-silicates  by  the 
ordinary  solvents,  while  silicates  can  be  decomposed  and  investi- 
gated either  by  the  methods  of  Borichy  or  Behrens.  Both  methods- 
rest  upon  the  recognition  of  the  forms,  etc.,  of  artificially  produced 
crystals. 

The  size  of  a  fragment  for  testing  may  vary,  according  to  cir- 
cumstances, from  that  of  a  poppy  seed  to  a  pin  head.  Good  results 
are  recorded  from  fragments  of  not  more  than  0.2—0.7  sq.  mm. 

The  substance  to  be  tested  is  placed  on  glass,  protected  by  a 
film  of  balsam,  and  covered  with  a  spherical  drop  of  the  solvent,* 
which  should  be  allowed  to  act  until  all  the  different  elements 
composing  the  sample  are  in  solution  (in  the  case  of  a  very  small 
fragment  until  it  has  all  dissolved).  Transfer  the  solution  to 
another  protected  object-glass,  and,  after  evaporation,  the  crystal- 
lizations characteristic  of  the  different  elements  will  be  seen. 

If  the  evaporation  is  too  rapid  and  the  crystallizations  incom- 
plete, the  residue  should  be  redissolved  in  water,  or  a  very  dilute 
solution  of  the  solvent  employed,  transferred  to  a  fresh  glass  and 
allowed  to  recrystallize. 

Borichy 's  Method. f     (Hydrofluosilicic  Acid.) 

This  method  has  the  advantage  of  simplicity  of  manipulation 
and  relative  distinctness  in  results  ;  but  on  the  other  hand  these 
results  are  only  obtained  after  several  hours,  and  the  temperature 
has  an  influence  on  the  crystalline  forms  obtained.  It  is  well  to 
make  the  tests  in  a  temperature  of  about  i  5  °  C. 

Put  a  spherical  drop  of  pure  hydrofluosilicic  acid  |  on  the  frag- 

*  For  micro-chemical  work  the  reagents  should  be  applied  in  very  small  drops, 
which  spread  out  on  glass  to  discs  2  mm.  in  diameter.  For  manipulating  the  reagents 
use  platinum  wires,  0.5  mm.  in  thickness. 

■\  Elemente  einer  ncuen  cheiiiischniikroskopischeti  mineral-  itnd  Gesteins-aiialyse, 
Pragg,  1877. 

Translation  of  above  by  Winchell  in  Geol.  and  N^at.  Hist.  Survey  of  AUnn.,  Vol. 
XIX.,  Ann.  Report,  1890. 

J  The  strength  of  the  solution  should  be  about  3  '/^  %;  for  if  too  weak  many  minerals 
do  not  give  satisfactory  results,  and  if  too  strong  a  very  large  number  of  fluosilicate 
9 


I30  CHEMICAL   AND   MECHANICAL    TESTS. 

mcnt  and  leave  it  for  some  hours  in  damp  air  until  the  action  has 
been  sufficient,  then  transfer  it  to  a  dry  air  bell-glass  and  allow 
evaporation  and  crystallization  to  take  place. 

For  the  microscopic  examination  the  objective  (200-300  diams. 
best  for  these  observations)  can  be  protected  with  glycerine  and 
a  mica  disc  or  thin  cover-glass,  or  the  drop  can  be  all  evaporated 
and  the  crystals  covered  with  liquid  balsam  and  a  cover-glass. 

Crystallizations  Obtained  by  Borichy's  Method. 

Potassuini.  From  hydrofluosilicic  solutions,*  isotropic,  colorless 
crystals  of  K2SiFlg,  in  cubes,  octahedra  or  combinations  of  these 
forms  with  the  rhombic  dodecahedron.  Apparently  orthorhombic 
crystals  may  form  from  acid  solutions  and  at  a  low  temperature, 


e  9  D  (J 

Fig.  78.  —  Fluosilicate  of  potassium,  t  Fig.  79.^Fluosilicate  of  sodium,  f 

but  if  these  crystals  are  dissolved  in  hot  water  and  recrystallized 
they  will  assume  the  normal  forms. 

Platinic  chloride  will  produce  under  proper  conditions  sharp, 
yellow  octahedra  of  K^PtClg. 

Sodhmi.  From  hydrofluosilicic  solutions,  colorless,  very  weakly 
doubly  refracting,  hexagonal  crystals  of  Na^SiFlg,  which  are  gen- 
erally longer  the  higher  the  percentage  of  calcium  in  the  solution. 
This  test  is  very  certain  even  for  small  amounts. 

Calcium.  From  hydrofluosilicic  solutions,  monoclinic  crystals 
of  CaSiFlg  -f-  2H.,0  of  various  forms,  generally  spindle-shaped, 
with  not  very  strong  double  refraction.  The  crystals  have  seldom 
straight-edged  boundaries  and  are  often  grouped  in  rosettes.  The 
addition  of  dilute  H2SO^  decomposes  the  crystals,  recrystallization 

crystals  are  formed  together  with  the  separation  of  much  silica,  thus  making  it  impos- 
sible to  carefully  differentiate  the  crystals  with  a  microscope. 

*  As  most  of  the  rock-forming  minerals  that  would  be  investigated  are  silicates,  the 
hydrofluosilicic  solutions  can  also  be  obtained  by  treatment  with  HFl. 

f  After  Levy  and  Lacroix. 


BORICHY'S  METHOD.  131 

yielding    long    prismatic     crystals    of    gypsum    (distinction    from 
strontium). 

Treatment  with  HFl  and  dilute  H^SO^  (in  excess),  producing  on 
evaporation  characteristic  crystals  of  gypsum,  fm-nishes  a  very 
delicate  test  for  small  percentages  of  calcium. 


Fig.  80.  — Fluosilicate  of  calcium.  *         Fig.  81.  — Fluosilicate  of  magnesium.* 

Magncshim.  From  hydrofluosilicic  solutions,  rhombohedral 
crystals  of  MgSiFlg  +  6H2O  with  plane  faces  and  sharp  edges. 
The  crystals  are  colorless  and  strongly  doubly  refracting  with 
positive  optical  character. 

The  formation  of  struvite  crystals  (NH^MgPO^  +  6H2O),  of 
coffin-like  forms,  is  very  characteristic  and  takes  place  from  very 
dilute  solutions  (rendered  alkaline)  on  the  addition  of  a  grain  of 
salt  of  phosphorus  or  a  drop  of  sodium  phosphate. 

Iron.  From  hydrofluosilicic  solutions,  crystals  of  FeSiFlg 
-|-  6H2O,  which  are  isomorphous  with  those  of  magnesium  salts, 
with  the  same  optical  characters.  They  may  be  differentiated  by 
moistening  with  potassium  ferrocyanide  or  ammonium  sulphide,  in 
the  first  case  by  turning  blue,  in  the  second  case  black. 

Ahiniinhini.  From  hydrofluosilicic  solutions,  not  satisfactory  on 
account  of  the  gelatinous  formation. 

When  the  gelatinous  formation  is  obtained  by  the  action  of  hy- 
drofluoric acid  on  an  ahtminons  silicate,  the  staining  test  can  be 
used  to  distinguish  between  fine  grains  of  feldspar  and  quartz  or 
iolite  and  quartz. 

*  After  Levy  and  Lacroix. 


132  CREMICAL  AND   MECHANICAL    TESTS. 

Behrens'  Method.*     (Hydrofluoric  and  Sulphuric  Acids.) 

Tliis  method  depends  on  common  reactions  that  can  be  made 
rapidly,  but  has  the  disad\antage  of  being  rather  comphcated  and 
requiring  delicate  manipulation. 

The  tests  are  best  made  upon  about  ^^  mg.  of  powder  with 
HFl  (pure  and  fuming).  As  soon  as  the  fluorides  begin  to  dry 
treat  with  dilute  H.^SO^  and  warm  until  white  fumes  of  SO3  appear. 
In  this  way  the  HFl  and  SiFl^  are  driven  off  and  the  sulphates  are 
left.  This  part  of  the  test  can  conveniently  be  made  on  a  piece  of 
platinum  foil.      Add  excess  of  water  and  concentrate. 

Transfer  a  drop  to  a  clean  object  glass  and,  while  still  liquid, 
examine  it  with  the  microscope.  Do  not  use  a  cover-glass  over 
the  drop. 

Crystallizations  Obtained  by  Behrens'  Method. 

Potassium.  Add  a  little  platinic  chloride  when  octahedral  crys- 
tals of  KjPtClg  (size  .18  to  .30  mm.)  will  appear,  which  are  clear, 
bright  yellow  in  color  with  strong  refraction. 


Fig.  82.  —  Potassium  platinic  chloride,  f    Fig.  83. — Sulphate  of  calcium  (gypsum ).f 

Sodium.  Use  sulphate  of  cerium  and  allow  a  small  amount  of 
this  reagent  to  act  through  a  capillary  pipette  upon  a  drop  of  the 
solution.  Very  small  aggregates  of  brown  crystals  (size  .02  mm.) 
of  the  double  sulphate  of  cerium  and  sodium  are  formed,  which 
are  clearly  visible  with  600  diams.  If  potassium  is  present  the 
double  sulphate  of  that  alkali  will  appear  in  larger,  grayish  grains 
(size  .05  to  .06  mm.).     An  excess  of  HgSO^  is  to  be  avoided. 

*  Naturkimde,  Amsterdam,  2,  Vol.  XVII.,  1881.  Chemical  News,  Vol.  LXIII.,. 
No.  1647,  June,  1891,  et  seq. 

Micro-chemical  Analysis,  Behrens  (Judd),  IMacmillan  &  Co.,  London,  1894. 
f  After  Levy  and  Lacroi.x. 


BEHRENS  METHOD.  133 

Calcium.  After  a  few  minutes  little  gypsum  crystals  (CaSO^ 
+  2H„0)  will  appear.  In  strongly  acid  solutions  the  thin  acicular 
crystals  will  be  grouped  in  bushes  or  stars  ;  in  neutral  solutions 
the  cr>'stals  will  have  the  normal  shape  of  selenite  crystals  or  form 
swallow-tailed  twins. 

Magnesium.  Use  salt  of  phosphorus  dissolved  in  water  and 
allow  it  to  mix  with  the  solution  (to  which  has  been  added  ammo- 
nium chloride  and  ammonia)  through  a  capillary  pipette.  From  a 
solution  containing  more  than  5  per  cent,  of  magnesium  are  first 
deposited  X-shaped  skeletons  and  rudimentary  crj-stals  of  Mg.- 


Fig.   84. — Magnesium  ammonium  phosphate.*         Fig.   85.  —  Caesium  alum.* 

NH^.PO^  -|-  6H,0.  If  the  solution  is  more  dilute  beautiful,  sharp, 
hemi-morphic  crystals  (.10  to  .20  mm.  in  size)  of  the  orthorhom- 
bic  system  will  appear.  These  crystals  often  resemble  the  roof  of 
a  house.  The  formation  of  the  crj^stals  is  assisted  by  heat.  Iron 
and  manganese  phosphates  yield  crystals  of  the  same  type,  but 
the  iron  is  separated  on  the  addition  of  ammonia. 

Aluminium.  Use  chloride  of  caesium.  Take  a  drop  of  the 
solution,  with  excess  of  HgSO^  driven  off,  and  touch  it  with  a 
platinum  w'ire  that  has  been  dipped  in  the  melted  chloride  of  caesium. 
Large  crystals  (.40-. 90  mm.  in  size)  of  caesium  alum  will  form, 
which  are  octahedral  and  cubo-octahedral  in  shape.  Iron  does  not 
interfere  as  its  crystallization  would  take  place  much  more  slowly. 
The  solution  should  not  be  too  concentrated. 

Special  Tests. 

Distinction  between  haiiynite  (contains  CaSO^)  and  noselite  (con- 
tains Na.,SOJ.  Treat  with  HCl  and  on  evaporation  the  character- 
istic crystals  of  gypsum  will  be  seen  if  the  mineral  is  haiiynite. 
Dilute  acid  should  be  used  and  as  low  a  temperature  maintained 

*  After  Levy  and  Lacroix. 


134  CHEMICAL   AND   MECHANICAL    TESTS. 

as  possible,  otherwise  crystals  of  anhydrite  would  form  instead  of 
gypsum. 

Recognition  of  apatite  by  test  for  phosphorus.  Treat  with  a 
drop  of  ammonium  molybdate  dissolved  in  HNO3.  After  com- 
plete action  remove  the  solution  to  a  clean  object  glass,  when  after 
slight  warming  a  large  number  of  very  small  yellow  crystals 
(rhombic  dodecahedral  in  shape)  will  form.  The  test  may  be 
used  to  distinguish  this  mineral  from  nephelite,  melilite  and  natro- 
lite.  In  the  presence  of  soluble  silica  evaporate  to  render  it  in- 
soluble and  treat  again  with  HNO3  and  the  reagent. 

Other  micro-chemical  tests  are  not  mentioned  for  the  reason 
that  in  elaborate  chemical  .investigations  of  sections  or  isolated 
fragments  recourse  should  be  made  to  the  most  complete  publica- 
tions on  the  subject. 


APPENDIX. 


Brief  Scheme  of  Classification  into  Systems   by 
Optical  Determinations. 


HOMOGENEOUS 


Isotropic 


Amorphous 

Isometric    . 

Anisotropic  . 


Uniaxial 


Tetragonal    . 
Hexagonal    . 

Biaxial 

Orthorhombic 

Monoclinic    . 

Triclinic  .   .   . 
AGGREGATE . 


The  whole  substance  shows  the  same  optical 
character,  except  in  the  case  of  twin  crystals 
when  the  different  portions  of  the  twin  are  af- 
fected differently. 

All  sections  of  the  substance  remain  dark 
during  a  complete  rotation  between  crossed 
nicols,  and  no  interference  figure  is  produced 
by  convergent  light. 

Absence  of  crystalline  form  or  cleavage. 

Presence  of  crystalline  form  or  cleavage. 

Sections  generally  show  some  interference 
color  and  extinguish  four  times,  at  90°  apart, 
during  complete  rotation. 

Determined  by  character  of  interference  fig- 
ures obtained  by  convergent  light  from  sections 
which  remain  dark  or  nearly  so  during  complete 
rotation. 

All  sections  show  parallel  or  symmetrical  ex- 
tinction. 

Sections  giving  interference  figures  are  four- 
or  eight-sided,  or  show  rectangular  cleavage. 

Sections  giving  interference  figures  are  three-, 
six-  or  nine-sided,  or  show  cleavage  lines  inter- 
secting at  60°. 

Determined  by  character  of  interference 
figures  obtained  by  convergent  light. 

Extinction  is  parallel  or  symmetrical  in  all 
sections  parallel  to  a,  b  and  c .  Color  distribu- 
tion is  symmetrical  to  two  lines  and  to  the 
central  point,  see  p.  46. 

Extinction  is  only  parallel  or  symmetrical  in 
sections  parallel  to  the  ortho  axis  b ;  all  other 
sections  show  extinction  angles.  Color  distri- 
bution is  only  symmetrical  to  one  line  or  to  the 
central  point,  see  p.  46. 

Extinction  angles  in  all  sections,  although 
in  some  minerals  these  angles  may  be  very 
small.  No  symmetry  in  color  distribution,  see 
p.  46. 

Not  homogeneous,  but  made  up  of  an  aggre- 
gation of  individuals,  all  extinguishing  at  dif- 
ferent times. 

135 


136 


APPENDIX. 


J,  Jt  i 


J.  04^ 


0.287 
0.179 

o  172 
o.  116 
0.072 
0.062 
0.058 
0.056 

0.050 

0.041 
0.040 

0.038 
0.036 
0.036 
0.034 

0.029 
0.027 
0.027 


Riitile 
Dolomite 
Calcite 
Titanite 


Double  Refraction  (maximum.)* 

^004   0.024  Hornblende  (common)  o.oio  Serpentine 


0.009  Corundum 

0.009  lolite  (Cordierite) 

0.009  Ouartz 

0.008  Kaolin 

o.ooS  Lahradorite,  Abj  Ahj 

0.008  Oligoclase,  Ab^  Ahj 

0.008  Albite 


0.024  Diallage 
<J^i2»'o.022  Augite 
^<>ty'o.022  Tourmaline 
Hornblende  (baJ?£<Ki^  0.021  Sillimanite  (iMbrolite) 
Zircon  j    i^*^  0.021  Anthophyllite 

Biotite  5,  <^/)f  0.020  Glaucophane 

Epidote,  dark  ^  IV Ik  0.016  Cyanite  (Disthen*^* 
'"  ■  0.013  Hypersthene         %Jf  40.007  Orthoclase 

0.013  Scapolite  (Marialite)     0.007  Microcline 

0.013  Anorthite  0^*^0.005  Zoisite 

^0.012  Xatrolite  aJP/'o.oo4  Nephelite  (Elseolite) 

o.oii  Chlorite  (Clinochiore)  0.003  Apatite 

o.oil  Andalusite  g  at^o.003  Melilite 

O.OIO  Staurolite 

O.OIO  Gypsum 


Talc 

Muscovite 

-Egirite 

Epidote,  light       %m 

Chrysolite  (Olivine) 

Scapolite  (Meionite 

Phlogopite 

Diopside 

Actinolite 

Tremolite 


4  9'9^o.o\o  Topaz 
A  iff  °  °i°  Enstatite 


0.002  Vesuvianite 
0.002  Tidymite 
J^O/0.002  Leucite 

o.ooi  Chlorite  (Penninite) 


Indices  of  Refraction  (mean).' 


2.712  Rutile 

1.679  Chrysolite  (Olivine) 

1.564  Phlogopite 

2.38    Perofskite 

1.665  Enstatite 

1.56     Serpentine 

2.00    Spinel  (Chrome) 

1.664  Sillimanite  (Fibrolite) 

1.559  Labradorite,  Abj  Anj 

1.95     Zircon 

1.656  Glaucophane 

1.55 1  Scapolite  (Marialite) 

1. 94 1  Titanite 

1.642  Hornblende  (common) 

1.55     Kaolin 

1.792  .Egirite 

1. 641  Anthrophyllite 

1.547  Quartz 

1.78     Allanite  (Orthite) 

1.638  Andalusite 

1. 54 1  Nephelite  (Elseolite) 

1.78    Garnet  (Almandite) 

1.635  Apatite 

1. 54 1  Oligoclase,  Ab^  An^ 

1.767  Garnet  (Grossularite) 

1.635  Tourmaline 

1-54  -r  Canada  balsam 

1.766  Corundum 

1.630  Melilite 

1.539  lolite  (Cordierite) 

1.75 1  Epidote 

1.622  Topaz 

1.535  Albite 

1.75     Garnet  (Pyrope) 

1.622  Dolomite 

1.525  Gypsum 

1. 741  Staurolite 

1. 62 1  Actinolite 

1.523  Microcline 

1.723  Hypersthene 

1. 62 1  Tremolite 

1.523  Orthoclase 

1. 72 1  Vesuvianite 

1. 618  Biotite 

1.509  Leucite 

1.72     Spinel 

1. 60 1  Calcite 

1.503  Haiiynite 

1.720  Cyanite  (Disthene) 

1.588  Chlorite  (Clinochiore) 

I.48S  Analcite 

1. 719  Hornblende  (basaltic) 

1.586  Muscovite 

I.4S3  Natrolite 

1. 715  Augite 

1 .583  Anorthite 

1.483  Sodalite 

1.708  Zoisite 

1.582  Scapolite  (Meionite) 

1.477  Tridymite 

1.688  Diallage 

1.576  Chlorite  (Penninite) 

1.46    Opal 

1.683  Diopside 

1.572  Talc 

*  Wei nschenk's  Tables,  Die  gesteinshildcnden  Minera/ien,   1901. 


Diagram  showing  relation  between  strength  of  Double  Refraction, 
Interference  Colors  and  Thickness  of  Section.* 


■^ 

,^ 

- 

li 

\^ 

\ 

^ 

^~^ 

^ 

^ 

■ 

w 

\^ 

\ 

\ 

\ 

^ 

\ 

^^ 

WW 

\  \ 

\ 

v 

"^ 

^ 

\v 

^\\ 

\^ 

\ 

\ 

t 

^ 

1 

\\< 

\\ 

\ 

\ 
\ 

\ 

^ 

\. 

^ 

• 

n 

\\\\ 

\  \ 

\ 

\ 

\ 

^ 

^. 

\ 

\\\\^ 

\\\\ 

\\' 

\^ 

\ 

^ 

\ 

^^ 

\\\ 

^\\\\ 

\\ "" 

\\ 

\ 

\ 

\ 

N, 

\\\ 

\\\\ 

\\\ 

\     \ 

\ 

\ 

\ 

s 

N, 

, 

\\\ 

\\\\ 

\\\ 

\  \ 

\, 

\ 

\, 

s. 

N,  ^ 

\\\ 

\\v 

w^ 

^  \ 

s^  \ 

V 

\. 

\ 

•v 

\ 

\\ 

\\ 

^\\\ 

\\ 

\ 

\ 

\ 

\ 

• 

\  \ 

\  \  * 

\\\ 

\  \ 

\    \ 

\ 

\ 

\, 

" 

\\\ 

w 

\\\^ 

\  \ 

\ 

\ 

\ 

\^ 

\ 

" 

\\ 

w 

w 

\  \ 

.\ 

\ 

\, 

\ 

\\ 

,\ 

\\\ 

\\  \ 

\ 

N. 

\ 

\ 

\ 

\\ 

\\\ 

\\ 

\ 

\ 

\ 

\, 

■ 

\ 

\  \ 

\   ^ 

\  \ 

,   \ 

\     \ 

\ 

\, 

V 

\  \ 

\ 

\\  \ 

\    \ 

\ 

\, 

\, 

\, 

N.  ^ 

y 

\ 

\  w 

\  \ 

\ 

\ 

\ 

\ 

\ 

\ 

\  \ 

w 

\  \ 

\     \ 

\, 

\, 

\, 

\\ 

\^ 

A 

w 

\  \ 

v^ 

V 

\, 

\, 

\ 

\ 

\  \ 

I  \  \ 

\ 

V 

\, 

s, 

\ 

\ 

\ 

\  \ 

\\ 

y   \ 

\ 

\ 

\ 

s,  ■ 

\ ' 

\ 

\^ 

\^ 

^\ 

\ 

\ 

\, 

\ 

\ 

\\ 

\ 

\\ 

\\ 

\ 

\ 

\ 

\ 

' 

\ 

\ 

\\ 

\ 

\\ 

A' 

\  \ 

\ 

\ 

\ 

\: 

\ 

\ 

i^ 

^ 

\ 

\ 

\ 

\ 

\ 

N 

Iron  Gray. 
Pale  Gray. 


Pure  White. 
OO05. 

Pale  Yellow. 
Bright  Yellow. 

Orange  Red. 


■q  Red. 

Violet  (sensitive  tint  No.  1). 
0010. 

Blue. 

Bluish  Green. 
Green. 

Yellowish  Green. 
0015. 

Yellow. 

Orange  Red. 

Red. 

Violet  (sensitive  tint  No.  2). 

0-020-Blue. 
Bluish  Green. 

Green. 

Yellowish  Green. 
Yellow. 

0-025-RoseRed. 
Reddish  Carmine. 

■^  Purplish  Carmine. 

-  Violet  Gray  (sensitive  tint 

-^  Bluish  Gray         No.  3). 

0"030— Brownish  Green. 


Pale  Green. 


Pale  Gray. 


0035. 


0040. 


o  o 


9  9-99 

ti  «     "-     >i 

o  «     w     © 

o  o    o    c 


©  ©  ©  © 

©  ©  ©  © 

"»  ©  ©  wl 

©  Vi  ©  sn 


©  © 

c  6 

&i  lb 

©  <^ 


*Pirsson  and  Robinson,  Aw.  Jour.  Set.,  iv,  Vol.  X,  Oct.,  1900. 


138  APPENDIX. 

Order   of   Consolidation  of    the  Constituent    Minerals   in 

Plutonic  Rocks. 

"  There  is  in  plutonic  rocks  a  normal  order  of  consolidation  for 
the  several  constitutents,  which  holds  good  with  a  high  degree  of 
generality.  It  is  in  the  main,  as  pointed  out  by  Rosenbusch,  a 
law  of  '  decreasing  basicity.'      The  order  is  briefly  as  follows  : 

"  I.  Minor  accessories  (apatite,  zircon,  sphene,  garnet,  etc.)  and 
iron  ores. 

"  2.  Ferro-magnesian  minerals  —  olivine,  rhombic  pyroxenes, 
augite,  aegirine,  hornblende,  biotite,  muscovate. 

"3.  Felspathic  minerals  —  plagioclase  felspars  (in  order  from 
anorthite  to  albite),  orthoclase  (and  anorthoclase). 

"4.   Quartz,  and  finally  microcline. 

"  In  most  rocks  such  minerals  as  are  present  follow  the  above 
order.  The  most  important  exceptions  are  the  intergrowth  ot 
orthoclase  and  quartz  and  the  crystallization  of  quartz  in  advance 
of  orthoclase  in  some  acid  rocks,  and  the  rather  variable  relations 
between  groups  2  and  3  in  some  more  basic  rocks.  The  order  laid 
down  applies  in  general  to  parallel  intergrowths  of  allied  minerals  ; 
thus  when  augite  is  intergrown  with  aegirine  or  hornblende  the 
former  mineral  forms  the  kernel  of  the  complex  crystal  and  the 
latter  the  outer  shell  ;  when  a  plagioclase  crystal  consists  of  suc- 
cessive layers  of  different  compositions  the  layers  become  progres- 
sively more  acid  from  the  centre  to  the  margin. 

"  Certain  constituents  having  variable  relations  are  omitted  from 
the  foregoing  list.  Thus  nepheline  (elaeolite)  and  sodalite  belong 
to  group  3,  but  may  crystallize  out  either  before  or  after  the 
felspars."  * 

*Harker,  Petrology  for  Students,  p.  28,  1895. 


OPTICAL    SCHEME. 
Introduction. 

The  scheme  is  designed  to  furnish  the  student  with  a  practical 
method  of  recognizing  the  common  minerals  in  rock  sections. 

The  arrangement  followed  has  been  to  group  minerals  having 
general  optical  characters  in  common,  at  the  same  time  giving 
their  specific  characters  so  as  to  make  it  possible  to  distinguish 
one  from  another.  In  each  rectangle  the  minerals  are  arranged  in 
order  of  their  indices  of  refraction. 

A  tabulation  of  the  minerals,  with  a  list  of  optical  characters 
appended,  is  of  aid  to  the  skilled  investigator,  but  of  very  little 
assistance  to  the  beginner. 

The  more  common  minerals,  or  those  which  are  important  petro- 
graphically,  are  printed  in  heavy^-faced  type  ;  the  minerals  of  less 
importance  in  small  capitals. 

Abbreviations  and  Conventions  Used. 

A.  ==  Amorphous. 

I.  :^  Isometric  system. 

T.  ^  Tetragonal  system. 

O.  =  Orthorhombic  system. 

M.  =  Monoclinic  system. 

Tri.  =Triclinic  system. 

H.  =  Hexagonal  system. 

M(H).  =  Monoclinic,  with  hexagonal  form  or  characters,  as  in  the  case  of  biotite. 

±  ^=  At  right  angles  to. 

El.  =  Elongation. 

Ex.  ^=  Extinction. 

II  Ex.  =r  Parallel  extinction,  as  when  the  crystals  extinguish  parallel  to  cleavage 
lines  or  crystal  edges.  Extinction  which  is  symmetrical  to  intersecting  cleavage  lines 
is  also  included  under  this  term. 

The  refractive  indices  are  printed  in  heavy-faced  type. 

The  term  "grains"  is  used  to  describe  not  only  minerals  which  occur  in  typically 
granular  form,  but  also  those  which  have  coarser  allotriomorphic  form,  as  elaeolite  and 
sodalite  in  plutonic  rocks,  such  as  syenite,  etc. 


139 


I40  OPTICAL   SCHEME. 

General  Rules  for  Use  of  Scheme. 

The  division  of  the  scheme  into  two  vertical  columns  is  based 
on  the  values  of  the  refractive  indices,  as  determined  by  the  "  relief" 
and  appearance  of  the  surface.  When  the  refractive  index  is  above 
1.60,  the  relief  is  fairly  well  to  distinctly  marked  and  the  surface 
rough  to  very  rough,  depending  on  the  value  of  the  index.  Most 
of  the  rock  forming  minerals  with  indices  below  1.60  show  no 
relief  and  a  smooth  surface,  except  in  the  case  of  a  few  of  the 
rarer  minerals  (mostly  isometric),  which  have  very  low  indices  and 
hence  rough  surface. 

Mistakes  may  easily  be  made  in  the  case  of  minerals  near  the 
limit ;  but  practice  and  the  use  of  the  Becke  test  should  soon  make 
possible  the  classification  into  the  two  groups  suggested  by  the 
scheme,  and  the  appended  descriptions  will  help  to  check  errors. 

When  an  unknown  mineral  lies  adjacent  to  one  that  is  known, 
use  the  Becke  test  for  obtaining  the  relative  refractive  index  ^Df  the 
unknown  mineral  :  focus  sharply  on  the  line  of  contact  between 
the  known  and  unknown  minerals,*  then  raise  the  objective  slightly 
and  the  "  bright  line  "  will  appear  on  the  side  of  the  mineral  having 
the  higher  index. 

The  horizontal  divisions  of  the  scheme  depend  on  the  relative 
strength  of  the  double  refraction  based  on  the  observed  interfer- 
ence colors.  These  colors  to  be  of  use  in  classification  must  be 
correctly  recognized.  The  lower  and  middle  colors  of  the  i  ° 
order,  from  bluish-gray  through  white  to  yellow,  are  easily  known. 
The  bright  red,  blue,  green,  etc.,  colors  of  the  i°,  2°  and  3° 
orders,  can  also  be  differentiated  without  trouble  from  the  very 
high  order  colors  (4°  and  above),  which  are  essentially  white  in 
tone  with  no  decided  color  tint. 

When  confusion  arises,  the  exact  order  of  the  color  can  be 
determined  by  a  quartz-wedge,  as  given  on  p.  34.  Furthermore, 
a  ],^  undulation  mica-plate  serves  to  quickly  distinguish  between 
the  I  °  order  white  and  the  practically  colorless,  high  order  tint  of 
calcite,  titanite,  etc. ;  as  after  the  insertion  of  the  test-plate  the  i  ° 
order  white  suffers  a  marked  change  in  color,  while  the  very  high 
order  (practically  white)  tint  shows  no  appreciable  change. 

*  Have  the  convergent  lens  or  condenser  lowered  and  the  analyzer  out  during  this 
test. 


GENERAL   RULES  FOR    USE    OF  SCHEME.  141 

In  the  determination  of  these  interference  colors  care  must  be 
given  to  considerations  of  orientation  and  thickness.  The  section 
must  give  the  maximum  interference  color  of  all  the  obtainable 
sections  of  the  mineral  in  the  rock-section.  Such  sections  will  be 
parallel  to  the  c  axis  in  the  uniaxial  minerals  and  to  the  axial  plane 
in  the  biaxial  minerals  ;  and  will,  therefore,  in  convergent  light 
never  show  the  emergence  of  an  optic  axis  or  bisectrix.  Crystal 
"form,"  cleavage,  pleochroism,  etc.,  may  at  times  aid  in  the  selec- 
tion of  these  sections.  The  thickness  of  the  section  must  also  be 
considered,  and  it  is  well  in  all  cases  to  pick  out  some  known 
mineral  in  the  section,  as  quartz,  and  note  its  maximum  interference 
color.  Knowing  how  this  varies  from  the  color  given  by  the 
scheme  for  a  section  0.03  mm.*  in  thickness,  due  allowance  can 
be  made  for  a  like  variation  in  the  colors  given  by  the  other 
minerals. 

Under  the  subhead  of  pleochroism,  the  vibration  direction  of  the 
ray  of  a  definite  color  is  given  and  not  the  direction  of  transmission 
of  that  ray. 

The  interference  figures  in  convergent  light  increase  in  clear- 
ness and  distinctness  with  the  strength  of  the  double  refraction. 
In  uniaxial  crystals  sections  at  right  angles  to  the  optic  axis,  /.  e,, 
sections  which  remain  dark  during  a  revolution  between  crossed 
nicols,  show  the  best  interference  figures.  In  biaxial  crystals  the 
most  characteristic  interference  figures  are  shown  by  sections  at 
right  angles  to  the  acute  bisectrix. 

Crystal  sections  which  are  too  small  do  not  giv^e  very  satisfactory 
interference  figures  with  convergent  light. f 

Any  scheme,  however  designed,  makes  a  more  or  less  arbitrary 
classification  of  the  minerals,  and  when  in  doubt  it  is  always  safer 
to  look  for  the  mineral  on  both  sides  of  the  scheme  line. 

The  rarer  minerals  are  not  included  in  this  scheme,  so  when  the 
determination  of  a  mineral  is  uncertain  or  not  positive  recourse 
should  be  had  to  more  elaborate  tables. 

*  The  minerals  are  grouped  according  to  the  maximum  interference  colors  given  by 
sections  of  the  thickness  of  0.03  mm. 

f  In  Seibert  microscope,  with  No.  V.  objective,  the  results  are  unsatisfactory  unless 
the  crystal  section  takes  up  more  than  i  or  i  of  the  field  of  view. 


of  the  Common  Minerals  in  Rock  Sections. 

ess  Section  =  0.03  mm.) 


SECTION   IS   OPAQUE. 


H.  Hematite.  H.  Graphite.                               ,  „•    a  i       ^, 

iften   showing        Black,  metallic  scales  and  minute  grains.  Minute  particles  or  black,  metallic  flakes  or 

rains.                          Also  red  witiiout  metallic  lustre,  or  trans-  grains, 
parent  in  red  tints.    May  be  earthy. 

ular  masses.        H.  Umenite  (Menaccanite). 

Black,  metallic,  irregular  masses,  often  sur- 


LiBH;SKT 

lfMlV£RSIiy  CF  ^ 


Scheme  for  the  Optical  Determination  of  the  Common  Minerals  in  Rock  Sections. 

(Average  Thickness  8ectioii  =  o.o3  mm.) 


I.  Hanietltfl. 


THE 

I.  Pyrlte. 


SECTION   IS   OPAQUE. 


LXQHT  IS  TRAN8MXTTED.    ISOTROPIC. 


n<1.60. 


OPAL.  1.46.  I.HAl}T!ctTK(Ha07De}ll.D03.  I.Spihbl.  1. 

Inrlew  MtchM  or  »elu8.    M»j-  hB«  M    I.  Nosklitb  (NmmdI    f      ,.     ..      ^    .    ,1     CoIoHm.  t 
llraw  spWrulltlc  Btniclore.    Surface  ap-       Colorlwjj,  bluish  or  yellowish,  dodecaliedral  ||        ory»talB  o 

,„ „    ,   ...  margioally   or   reRularly  nmoged.    Sur- .1         mal.     Sui 

IRALCITE.   1.488.        ,  „     ,  ..  racesUghUr  rougS.    AlCeratlo a  common.    I  , .  _   __   ,    ,  --«  .    ,  a.c 

condary,  colorless  iralDB.     Surface  r  a  the  r  UiitlnBulahea  br  micro^bemlcal  tesU         ,,  I.*  OaniBt.  l.TBO  to  1.8S6. 

rough.    Cubical    c^kTage    way   be    seen.  i  ""nguisneu  ny  micr  cbh««.  Nearly  colorlesa  to   reddUh.dodt 

" -•-■'    -  '     ■'^  I.'Leuctte.  1.609.  elc-cryalilaorgrmln..    Irreguli 

Small,  rounded,  colorleu  crystalK    Indu-  Surflue  rery  rough, 

sious     regularly     arranged,     (lenerally 


iways  optically  nor- 


;rg," 


ZiIGHT  IS   TRANSMITTED.    ANISOTROPIC. 

»'  <  1.60.     No  Maiked  Relief  and  Smooth  or  Slightly  Rough  Suiface.         n'  >  1.60.     Medium  to  Strong  Relief  and  Rough  or  Very  Rough  Surface. 

. 

U(H)  Ohlorlte    1  BT6    (==) 

u  AVI.MIS1TE.  1£3B    (-)     El   W               O  STAUBOUTB.  1741    (y)     E\  li  .■ 

lluBUl9tirroin«er|.cuilDe. 

^ 

0.  HTPOfBtbene.  1.733-  (-).    El.  (  <'.               Brown,  colitiunar  cryjtaU  or  gralue.    Very 
Brownish,  ahurt  prismatic  crystaln  or  grains.          rough  surface.  Strong absorptl on.  Lamet- 
Clearage  |  plnacolds  and   prism  of  9*2°.            lar  twinning.    SomellmBs  surniuuded  by 

s 

£     > 

1  Ei. 

§    s 

H  r*  TniDYuiTR   1477 

Til.  FlaEloclaBe.  I.G36  m  1.683.  (*:).  El- 
(  i,  almie  Iwln  lamellie]  gen.  I  *'. 
Like   onhoclase,   but   with   polysyotbellc 

Colorless,  abort  prisms  or  rods,  with  perfect        &»enUBllj  colorless,  columnar  cryHtats  or 
baaal  cleavage.     II  Ex.    Relief  uot   very          graius,  often   iotergrowa    with   epiduie. 

VijS 

crossed  dIcoIb. 

Tri    Anoiithoci,*SB. 

T  Mbmlitk   1830    {— )     El  11''                            anomalous.    I(  Ex.  (eseept  tn  cIln«»olalir,. 

4U 

often  ihowing'-peg  structure."     fttllei'       El.  II  .'. 

Aggregates  (often  radial )  of  culnrless,  Bbrous 

^i* 

. 

quarti.  1,  Ei. 

plate.   II  Ex.                                                          colors,  often  anoaiaious  and  xonally  nr- 
H.  Apatlf.  1.635.  I-).    E..I1.-.                           ™««*-   ""■ 

g«o 

^ 

Colorless  grains,  rarely  prisms  which  often 

£ifCS"t'Eir£}\iSB 

Bfe  5 

5 

ft^a"t\tnS"a"ny"'ry  t'l'u"^    "^""^'^  "' 

purllUB,    or    grains.     Belief    not    very       Colorless  cryslal*,  grains  or  plalea,  wblcU 
colors.    Often  as  inclusions  lu'oiher  mln-          PleochroUm  unh' nollced  when  the  color 

|h| 

s 

I.»  Leuolte.  I.BOB. 

H.  Nephelito  (Nepbcllne,  Elieolite).  1.841. 

1     5 

SLGYMCU.  1625.  (+),       .,      ,     .    . 

Colorless  grains,  oltan  with   columnar  or 

1 

OfWD  decomposed.     May  be  hard  to  dU- 
tlDgufsta  from  feldepar,  when  gelatlnlzar 

slons!'  II  Ex. 

Carlsbad  twlus  frequent.     Ofioa   dccom- 

H.  Quarti.  1.B47.  l  +  l. 
Colorlees  gruioi.      Xo  clearage.     Always 

Santdine  usually  fresh  and  sboiri  pinacoldal 
parUng. 

CKaleedony   baa  radially  Gttroua  jlruclure, 

Like  Orltaoetase,  but  wHb  "  oroised  "  twin, 
ning. 

S 

BrawD  or  green  shreds  (sections  ->-  lAtmlar       Colored,  radiate  aggregates  and  prlsmailc 

,2        n 

1        ^ 

1 

cleavage.  Surface  not  verv  rough.  Strung          of  elongation.    I!  Ex. 
l?r"rSn«»t^V,Ti;i?hmTb«"-"hl^^^^^    W.  E^ldote.  1.781    (^*^1^^^^^^ 

1  -mS 

s 

H.  Amphlbole   (Hornblende),     1.681    to    j,.  Aciiitk  (.Egirlr..  i    i  792               i 

s 

Colorless  to  green  to  brown,  prismatic  crya-       '*''*]?."?"  ^J^yj".}!^' ,'"  i',",,  ^,'  ■  J  "  '     i  '  1 

82" 

Jogs 
1  II  g| 

II^j 

■'SKLSrrri"",;::^!!: 

U.  UQBCOTlte.  1.DB6.  (-).  El.  1 II  oleaT.)  i| ,'. 

0.  SiLi-iMANtTC  1.664.  (  r).    El.  n<'.               H.   Pyroxene  (Auglte).    1,680  to   1.730. 
Colorless,   long,  slender    needle*,   often   In        (  +  ).     El.  lli'. 

1  S"   '   Eu        Colorless,  sc^y  aggregates'." '  Silghlly  rough 


i  ..s    B 

I  S  S  J      E 

I  AZ^° 


J.  OhryBOUte  (Olivine).  1.6T9.  I  +  ). 

sorption  common.  *  Ex.  angles  very  small.         '  yetlowlih  to  colorlwa,  we-lge-sli aped  cry »j 
Qo.iQo.  tals,  gr?'""  <"   One  aggregaiefc    ,1**""' 


Al-       Colorleuimav  be  bluish  and  ■poiiM),hifldt- 
llke  crystals  and   eolunnar  aggregates. 


H.Tltuilte(Spbene) 


r   light   colored  gralci 


polarised  light    Cleavage  11  R.     luterfer-  Included  In   other  minerals,  ofUn  sur- 

ence  colors  very  high  order.     Twin  lamel-  rounded  by  halo. 


AfgrcKatea:   Serpentine,  1.B6. 
Chlorite.  1.5T6.  ' 

7  {Kaolin] 


d  usually  pleoetarolc.    Scaly. 


irfercnce  ooloTe.  h' 
.  opaque,    Sealj.    Very  weak  doable  refVaotton. 

•  May  show  OpUeal  Anoma 


INDKX. 


MINERAL    NAMES    IN    HEAVY-FACED    TYPE. 


Abbreviations, ix 

Absorption, 25 

directions, 25 

tints, 17 

Acmite, 81 

Actinolite, 82 

Acute  bisectrix, 5 

^girine, 81 

^girine-augite, 81 

Aggregate  structure, 38 

Albite, 100 

AUanite, ...    93 

Allotriomorphic, 15 

Aluminium,   test   for,   by  Borichy's 

method, 131 

test  for,  by  Behren's  method,  .    .133 

Amorphous  bodies, 2 

Amorphous  substances,  test  for,  ...     26 

Amphibole, 81 

Analcite, 54 

Analyzer,    ....  10 

Andalusite, 70 

Anhedron, 15 

Anisotropic  character,   test  for,    ...     26 

Anisotropic  crystals, 2 

Anomalies,  optical, 26 

Anorthite,     100 

Anorthoclase, 107 

Anthophyllite, 85 

Antigorite, 108 

Apatite,     65 

Apatite,     recognition    of,    by    test    for 

phosphorus, 134 

Apparatus    for  petrographical     labora- 
tory,      121 

Appendix, 135 

Arfvedsonite, 85 

Augite, 77 

Axes  of  elasticity, 4 

I 


Axial  angle c 

determination  of, 45 

Axial   plane, r 

Basaltic  Hornblende, 82 

Bastite, 74 

Becke's  method  for  determining  relative 

values  of  refractive  indices,  .  .  19 
Behren's  method.      (Hydrofluoric  and 

sulphuric  acids), 132 

Bertrand   lens, n 

Biaxial   crystals, 4   e 

vibration  directions  in, 4 

Biaxial    interference   figures,    ....     42 

Biotite, 86 

Bisectrix,   acute, c 

obtuse, 5 

Borichy's  method.       (Hydrofluosilicic 

acid), 129 

Broken  or  strained  crystals, 15 

Bronzite, 74 

Calcite, 62 

Calcium,  test  for,  by  Borichy's  method,  130 
test  for,  by  Behren's  method,  .    .  133 

"Cap"  nicol, 10 

Carbonaceous  Matter,  59 

Carbonaceous  particles,  test  for,  .    .    .126 

Carbonates,  test  for, 124 

Cement, 116 

for   mounting, 119 

Cementing, 116 

Centering    stage, 10 

Chalcedony, 62 

Characters    observed    by,     convergent 

light, 38 

crossed  nicols, 25 

reflected  light, 13 

polarized  light,       24 

43 


144 


INDEX. 


Cliaiacters  of  opaque  minerals,  ...     13 

tran.sparent  minerals, 13 

Chemical  and  mechanical  tests,    .    .    .123 
Chemical    (micro)   reactions,    .    .    .    .129 

separation, 128 

tests  on  crystal   in  section,  .    .    .123 

Chiastolite, 70 

Chlorite  Group, 89 

Chromite 50 

Chrysolite, 74 

Chrysotile, 108 

Circular  polarization, 3 

Classifications  into  systems  hy  optical 

determinations, 135 

Clay, lOg 

Cleaning  and  finishing  sections,    .    .    .121 

Cleaning  microscope, 11 

Cleavage 22 

Clinozoisite,      93 

Color, 17 

diagram  (interference),     .    .    .    .137 

fringes, 46 

Complete  crystals, 13 

Condensing  lens, 9 

Consolidation    of  minerals  in  plutonic 

rocks,  order  of, 138 

Conventions, ix 

Convergent  light, 38 

characters  observed  by, 38 

Convergent  and  parallel  light,   resume 

of  uses  of, 47 

Cordierite,     76 

Corroded  crystals, 15 

Corundum, 60 

Crossed  nicols,  characters  observed  by,     25 

Crossed  twinning, 37 

Crystallites, 16 

Crystallizations    obtained    by  Behren's 

method, 132 

Borichy's  method, 130 

Cutting, 115 

Cutting  and  grinding  machines,    .    .    .112 
Cyanite, 108 

Damourite, 89 

Delessite, .    .    90 

Diagram,  showing  relation  between 
strength  of  double  refraction,  in- 
terference colors  and  thickness 
of  section, 137 


Diallage, 78 

Diamond  saws, 114 

Dichroite, 76 

Dilution  for  sjiecific  gravity  solutions,  .  127 

Diopside, 78 

Dipyre, 57 

Directions  of  elasticity, ix 

Dispersion, 5>  46 

Dislhene, 108 

Dolomite, 64 

Double  image, 2 

Double  refraction, 2,  26 

estimation  of  strength  of,  ...    .     29 
strength     of,     measured    by    von 

Federow  mica  wedge,    ....     34 
table    (maximum), 136 

Effects  produced  by  crystals  on  trans- 
mitted light, I 

Elaeolite, 67 

Elasticity,  axes  of, 4 

directions  of, ix 

Electric  motor  for  grinding  machines,  .  112 
Electro-magnetic  separation,  .  .  .  .128 
Emery,  grades  of,  for  grinding,    .    .    .118 

Enstatite, 72 

Epidote,      91 

Etched  figures, 124 

Extinction,       30 

angles, 4;  3° 

tests  for, 31 

wavy, 30 

Extraordinary  ray, 3 

Eye-pieces, n 

Faster  and  slower   rays,  test  for   vibra- 
tion directions  of, 32 

Fayalite, 76 

Feldspar  group, 95 

Fibrolite, 70 

Finishing  and  cleaning  sections,    .    .    .121 

Focusing, II 

Form,       13 

Fracture, 24 

Garnet, 51 

Gelatinizing  silica,  test  for, 124 

Glass  slides, 120 

Glaucophane, 85 

Graphite, 59 

Gridiron  twinning, 37 

Grinding, 117 


IN'DEX.  145 

Grinding  and  cutting  machines,    .    .    .112    Isotropic  character, 26,39 

apparatus  for  sections  with  parallel  crystals, 2 

faces, 113  w     1- 

,  ,  ,  ^  Kaolin, 109 

plates  or  laps, ii"?  t    ^      a     -^ 

_,  ^  Labradorite, 100 

Gypsum, 77  .  '  . 

„  ,  Laps  for  grinding, lie 

Gypsum  test-plate, ■12  ^        ., 

„     ,  r       •  \  Leucite, 52 

Hardness  of  grams, 126  t 

..  i^eucoxene, 59 

„   ..      . ' -*      Light,  ordinary, i 

Hauynite, 54  ,  ,    •    j 

-^  plane  polarized, i 

Hauynite  and  noselite,  micro-chemical  Limonite, 49 

distinction  between, 133  Lithia  Mica, 89 

Heating  sections  to  redness, 125 

Heavy  solutions, 127  Machines  for  cutting  and  grinding,  .    .112 

Hematite, tq  Magnesium,     test     for,     by     Borichy's 

Hexagonal  crystals, 4  method, 131 

High  relief 17  ^^^^  f°'''  ^J  Behren's  method,  .    .  133 

Hornblende, 81  Magnetic  iron  ore, 49 

Hyalosiderite, 76  Magnetic  pyrites, 49 

Hydrargillite, 109  Magnetite, 49 

Hydrofluosilicic  acid  tests, 129  Measuring  strength  of  double  refraction 

Hypersthene, 72  by  von  Federow  mica  wedge,  .     34 

Mechanical  and  chemical  tests,     .    .    .123 

Idiomorphic, 13  Melilite, ■ 58 

^^ocrase, 58  Menaccanite, 59 

llmenite, 59  Methods  of  preparing  sections  .    .    .    .111 

Immersion  method  for  determining  in-  Mica  ffrout)  86 


dex  of  refraction, 22 


Mica  plate  (quarter  undulation),      .    .     32 


Inclusions, 24   Mica  wedge,  von  Federow, 34 

Incomplete  crystals, 15    Micro-chemical  reactions, 129 

Index  of  refraction, ix,  17   Microcline, 99 

by  Becke's  method, 19   Microlites, 16 

by  immersion, 22    Microscope  (petiographical ),  ....  8 

by  Sorby's  method, 18   Microscopic  and  optical  characters  of 

Indicators   for  specific    gravity  separa-                       minerals  49 

°"' '^7    Minerals  and  thickness  of  section,  deter- 

Indices  of  refraction  (mean),  table  of,    136                ^lined  by  table  and  diagram,  .  35 

Interference    color,     determination    of          Monoclinic  crystals  4 

order  ot 34            principal  vibration  directions  in,  .  4 

Interference  colors, 29   Monoclinic  pyroxenes, 77 

Interference  figures, 38    Mounting, 119 

'^'^'^'^^> 42            solutions, 120 

""'^^'^1'      •    • 39  Muscovite, 86 

Investigation  of  microscopic  and  opti- 
cal characters  of  minerals,    .    .     13    Natrolite, 76 

lolite, 76   Negative  optical    character,  biaxial,     .  44 

Iron,  test  for,  by  Borichy's  method,    .  131             uniaxial, 40 

test  for,  by  Behren's  method,    .    .  133   Nepheline, .    .    .            67 

Isolating  crystals  or  mineral  fragments         Nephelite, 67 

for  testing, 126    Nicol  prism, 7 

Isometric  crystals, 2   Nosean, 54 


146  INDEX. 

Noselite, 54    Petrograpliical  microscope, 7 

Noselite  and    haiiynite,  micro-chemical  Phenocliryst, 14 

distinction  between, 133    Phlogopite, 86 

Nose-piece  for  microscope, 10    Picotite,      51 

Piedmontite,      92 

Objectives, 10   Plagioclases, 100 

Oblique   extinction, 30    Plane  polarized  light, i 

Obtuse  bisectrix, 5  Plates,  grinding,     ...             ....  115 

Oligoclase, 100    Pleochroism, 25 

Olivine, 74  test  for, 25 

Opal, 49   Pleonaste, 51 

Opaque  minerals,  characters  of,    .    .    .     13  Polarized  light,  characters  observed  by,    24 

Optical,   anomalies, 26  plane, i 

character,  biaxial,       44    Polarizer, 7 

character  uniaxial, 40  test  for  vibration  plane  of,      .    .    .       9 

classification  into  systems,  ....  135    Polishing  sections, 119 

distinctions  between  orthorhombic,  Polysynthetic  twinning,      .    .             •    •     37 

monoclinic  and  triclinic  sections  Positive  optical  character,  biaxial,    .    .     44 

(perpendicular  to  bisectrices),  .     46  uniaxial, 40 

distinctions  between  tetragonal  and  Potassium,  test  for  by  Borichy's  method,  130 

hexagonal  sections  (perpendicu-  test  for  by  Behren's  method,     .    .  132 

lar  to  optic  axis), 135    Preparation  of  sections, in 

and  microscopic  characters  of  min-  Principal  section,  optical, 3,4 

erals, 49  Principal  vibration  directions,  .    .    .    .  3,  4 

principal  section, 3,  4    Prism,  nicol, 7 

scheme, 139    Pseudomorphic  structure, 38 

Optic  axes,      4    Pyrite, 49 

Optic  axi.s, 3    Pyrites, 49 

Optics  for  optical  mineralogy,  ....       i    Pyroxenes,  monoclinic, 77 

Order  of  consolidation  of   minerals  in  Pyroxenes,  orthorhombic, 72 

plutonic  rocks, 138   Pyrrhotite, 49 

Order  of  interference  color,  determina-  Quarter-undulation  mica-plate,      ...     32 

tion  of, 34 

Ordinary  light, 1    ^J^^"^*^' ^° 

wedge, 33 

ray, 3 

Orthite, 93  Reflected  light,  characters  observed  by,    13 

Orthoclase, 95    Reflector, 7 

Orthorhombic  crystals, 4    Refraction,  double, 2,   26 

principal  vibration  directions  in,  .       4    Refraction,  index  of, ix,  17 

Orthorhombic  pyroxenes, 72    Relief,      17 

test  for, 17,  18 

Resorption  border, 15 

Resume  of  uses  of  parallel  and  conver- 


Parallel  and  convergent    light,  resume 

of  uses  of, 47 


Parallel  extinction, 30                .  i-  1.  ,_ 

'                                        -^                   gent  lignt, 47 

Parallel   face  sections,  grinding  appara-  t>    ^  ••        *  r       • 

'  *'            s    Kf"  Rotating  stage  of  microscope,  ....  9 

tus  for, 113  T»    i-i  rr 

'                                          -^  Rutile, 55 

Pargasite, 82 

Perofskite, 55  Sagenite,      56 

Perovskite, 55  Sanidine, 99 

Petrographical  apparatus, 121  Saussurite, 93,  107 


lADEX. 


147 


Saws, 114 

Scapolite  Group, 57 

Scheme  of  classification  into  sj'stems  by 

optical  determinations,  .    .    .    .135 

Scheme,  optical, 139 

Schiller  structure, 24 

Sections,  methods  of  preparing,    .    .    .111 

thickness  of  rock, 118 

Selenite-plate, 32 

Separation,  by  chemical  means,    .    .    .  128 

by  electro-magnet, 128 

by  specific    gravity, 127 

Sericite,      89 

Serpentine, 108 

Shagreened  surface, 17 

Shorl, 68 

Sillimanite, 70 

Simple  twinning, 37 

Single  image,  ....  2 

Skeleton  crystals, 17 

Slides,  glass, 120 

Slower  and   faster  rays,  test   for  vibra- 
tion directions  of, 32 

Sodalite, 54 

Sodalite  Group, 54 

Sodium,  test  for,  by  Borichy's  method,  130 
test  for,  by  Behren's  method,    .    .  132 
Sorby's  method  for  determining  index 

of  refraction, 18 

Special  micro-chemical  tests,     ....  133 
Specific    gravity  separation,   indicators 

for, 127 

Sph^erulitic  structure, 38 

Sphene, 93 

Spinel, 51 

Stage  of  microscope,  rotating,  ....       9 

Staurolite, 72 

Stauroscopic  methods, 31 

Strained  or  broken  crystals, 15 

Strength  of  double  refraction,  measure 

of, 29,  34 

Structure, 36 

Symmetrical  extinction, 30 

Systems,  classification  into,  by  optical 

determinations, 135 


Table,  double  refraction, 136 

indices  of  refraction, 136 

Talc, 91 

Test  for  vibration  plane  of  polarizer,  .    .  9 

Tetragonal  crystals, 4 

Thickness  of  rock  sections, 118 

Thickness  of  section  and  minerals,  de- 
termined by  table  and  diagram,  35 

Titanite, 93 

Transmitted  light,  characters  observed 

by, 13 

effect  on  crystals, i 

Transparent  minerals,    investigation   of 

characters  of, 13 

Tremolite, 82 

Triclinic  crystals, 5 

principal  vibration  directions  in,  .  5 

Tridymite, 62 

Topaz, 71 

Tourmaline, 68 

Twinning, 36 

Uniaxial  crystals,  vibration  directions  in,  3 

Uniaxial  interference  figures,    ....  39 

Uralite, 85 

Uralitization, 85 

Uses  of  parallel  and  convergent  light,  47 

Vesuvianite, 58 

Vibration  direction, 2 

directions  of  faster  and  slower  rays, 

test  for, 32 

directions,  principal, 3,  4 

plane  of  nicol, 9 

plane  of  polarizer,  test  for,    ...  9 

von  Federow  mica  wedge, 34 

Wavy  extinction, 16,  30 

Wedge,  mica, 34 

quartz, ^^ 

Wernerite, 57 

Xenomorphic, 15 

Zeolites, 77 

Zircon, 56 

Zoisite, 93 

a  and  /3, 93 

Zonal  structure, 37 


Date  Due 


mp  12  ^g 


SOUTHERN  REGIONAL  LIBRARY  FACILITY 


000  646  737     7 


University  of  California 

SOUTHERN  REGIONAL  LIBRARY  FACILITY 

305  De  Neve  Drive  -  Parking  Lot  17  •  Box  951388 

LOS  ANGELES,  CALIFORNIA  90095-1388 

Return  this  material  to  the  library  from  which  it  was  borrowed. 


